Farming for reagents can be a time consuming affair, and many find it more cost efficient to just buy them off the Auction House. But as profit margins for blacksmiths are already meager at best, some still like to farm for these hard-to-get components. Be aware that many of the mobs suggested are also quest objectives, and are at peak hours hard to get at. All of these are also in contested zones, so keep an eye out for gankers if you are on a PvP server. Remember that an alchemist buddy with transmuting skills can be of great aid when you are looking for essences of different kinds.
Fire Reagents
Elemental Fire. Needed for Greater Fire Protection Potion and Robe of Power as well as several other recipes in many professions. Drops off almost any fire elemental in the game. A good place to get them with minimal effort would be the Burning Exiles in Arathi Highlands. Can also be transmuted by alchemists from Heart of Fire.
Essence of Earth. Used in several plans. Drops from Greater Obsidian Elemental and Obsidian Elemental in the Burning Steppes, Desert Rumbler and Desert Rager in Silithus, Stone Guardians in Un'Goro Crater, Thundering Invader in Azshara, Infernal Sentry and Infernal Bodyguard in Felwood and elemental mobs in Molten Core.
Elemental Air. Used in Edge of Winter and Robe of Power. Drops off almost any air elemental. Good drop rates from Thundering Exiles in Arathi Highlands, and Whirlwind Ripper and Whirlwind Stormwalker in Desolace.
Globe of Water. Needed for several plans and quests. Can be fished from floating debris in Azshara, and drops from many water elementals from level 45+. Toxic horrors have them often. You can find some in the south west corner of swamp of sorrows at PureSpring Cavern.
Essence of Water. Needed for several plans. Drops from Toxic Horror in Felwood, Watery Invader in Winterspring, and Blighted and Plague elementals in Eastern Plaguelands. Moving between the Toxic Horrors and Watery Invaders seems like a fairly effective way to farm these.
Note: All of the above Water reagents may be fished, with high Fishing level, from the 'Patch of Elemental Water' nodes in Azshara. See this guide for more information.
Mote of Water. 10 of these will create you a Primal Water, needed for various healing enchants and other goods. These will drop off of Water Elementals in Outland (Nagrand/Shadowmoon Valley). Also can be fished from Pure Water nodes in Nagrand, which occur mostly in the Elemental Plateau.
Nature Reagents
Living Essence. Needed for several plans and enchants, such as those with +healing and Nature Resist attributes. Un'Goro Crater Tar pits, or Dire Maul East / West. Also off Warpwood and Irontree mobs in northern Felwood, or less reliably off the Decaying Horrors and Rotting Behemoths in the Weeping Cave in northeastern Western Plaguelands.
Lava Core and Fiery Core. Needed for several high-level plans. Only drops off elementals inside Molten Core. This has led the AH prices on some servers to be extremely high (up towards 300 Gold). Best bet to get these is to join a guild that raids Molten Core regularly.
A good place to get Core of Earth and Breath of Wind is across the southern Abyssal Sands in Tanaris. From The Noxious Lair in the west, head east staying north of the Dunemaul Compound until you are south of Broken Pillar. Then head southeast to the area immediately north of The Gaping Chasm. If you criss-cross a bit in said areas you should run into several Land Ragers and Gusting Vortexes on each round.
Almost everything used here comes from Herbalism. See down this page for things that can be farmed.
Vials of the various kinds are purchased at Alchemy vendors and Trade vendors. There is a vendor for the basic vials in almost every civilized area.
Mining
Copper and Tin:Loch Modan is a zone abundantly filled with both metals; tin is mainly found in the east half of the zone, with some concentrations to the northeast by the elite Ogre camp. The rather low number of mobs and their predictable behaviour makes this an excellent zone to farm Copper and Tin or level up your mining skill. This is an Alliance zone, but higher level Horde players can conveniently access it via the Badlands.
Mithril: Running a loop around the edge of the Hinterlands is one of the best ways to farm Mithril Ore, as the zone is rarely visited. Lethlor Ravine in the Badlands (a valley in the east of the zone with two entrances north and south) has been mentioned as a good place to farm Mithril, however, for 80s, the area is so small that the four or five nodes you will find will not have respawned until long after you found the fifth.
Thousand Needles since Cata is loaded with Mithril.
Farm any mob which drops green or better quality items. Named elite mobs are particularly good for this.
Run instances that are below your level over and over again. Instances have far more uncommon and rare drops than in the world.
The building skills (Tailor, Blacksmith, Leatherworker) can make green and above items.
People make lots of these items to raise their skill and then vendor them.
Make a deal with one/some of these people to give/sell/trade these items to you to melt down for enchanting raw materials.
For example, send cloth to a tailor. They can use it to gain skill and send you the greens they made for you to disenchant.
Take up Tailoring and make your own greens to disenchant. Black Mageweave gear commonly drops Dream Dust and uncommonly drops Nether Essences. If you're lucky, you may be able to buy crafted gear from other tailors for a cheap price, since lower-level tailored gear doesn't sell very well.
The Gatherer add-on is particularly useful for recording where you've encountered herbs in the wild.
Bloodvine
Bloodvine is the most rare herb in the game. It drops in Zul'Gurub when herbs are harvested by someone with a Blood Scythe in their bags, and sometimes from mobs.
Black Lotus
Black Lotus used to be the most valuable herb in the game found outside of instances, though they are largely supplanted by Frost Lotus now. Find it in places such as Winterspring and Eastern Plaguelands. It was required to create flask consumables until the advent of the Burning Crusade. They have a long respawn timer and are often heavily farmed.
Frost Lotus
Frost Lotus is the currently needed Lotus for Lich King endgame flasks. They have a chance to drop off *any* Lich King Herb, but do so with greater frequency with the higher level herbs, I.E. Lichbloom, Adder's Tongue, or Talandra's Rose. Auction prices are relatively stable on most realms between 20-40g per Lotus. Note: They are NOT millable by scribes, so are primarily useful to Alchemists.
You can also get Stranglekelp without being an herbalist, by fishing: small quantities of it turn up regularly in schools of fish and pools of floating wreckage.
Blindweed is a scarce high level herb that can be found in Swamp of Sorrows, Un'Goro Crater, and Zangarmarsh. Swamp of Sorrows is an excellent location for farming Blindweed because it has a very high drop rate and it is relatively out of the way.
Ghost Mushroom
Ghost Mushroom is a scarce mid-level herb (245) that is found in many caves throughout the upper level zones (level 45+) of Kalimdor and Eastern Kingdoms. Further, many caves in Outlands and Northrenddo contain a few of these herb nodes. They *are* millable by scribes, but due to the relatively scarce nature, they are not a great way of attaining pigment. Primarily used by Alchemists to make Ghost Dye which is needed for pre-BC relative endgame craftable tailoring items. Note that each node only gives 1 mushroom.
Netherbloom
Netherbloom was previously used in mana potions and is rare enough to present a high AH price. It requires a Herbalism skill of 360 and can be found all over Netherstorm.
Light Leather can be farmed pretty easily at the Maclure Vineyards and Stonefield Farm in Elwynn Forest for beginning Leatherworkers. It can also be obtained at the Raptor Ground in the Barrens.
A wide variety of low- to mid-level leather drops can be gotten at Whelgar's Excavation Site in the Wetlands. Go with a group for the quests there and you can easily fill your packs by the time you finish them.
The raptors in Whelgar's Excavation Site drop Light Leather, Medium Leather, Heavy Leather, Light Hides, Medium Hides and Heavy Hides. Most drops are in the medium range but approximately 25-50% will be a combination of light and heavy leather.
There are numerous cats and crocs in Northern Stranglethorn that drop Light Leather/Hide, Medium Leather/Hide.
Lvl. 25-26: The Growless Cave, at the base of Alterac Mountains are teeming with Yeti that drop medium and light leather. Used to be higher but now since Cata levels are lower.
Also the crocolisk island (the southernmost island in the Loch Modan) has a pretty fast spawn rate and every mob is skinnable for leather, which is mostly of the light variety.
The abundance of wolves (level 40+) in the usually scarcely populated Hinterlands provide both thick and heavy leather. They respawn quickly. It's rare to not find a wolf right outside of Aerie Peak.
Lvl. 45+:Badlands. Elite whelps in Lethlor Ravine can drop Black Scales but the drop rate is abysmally low.
Lvl. 58+: Upper Blackrock Spire. The dragonkin here drop scales from skinning at about a 10-15 percent drop rate. Also, scales of any color can be skinned off the chromatic dragonkin. Chromatics are found in the Rend encounter and at the end boss, General Drakkisath.
LvL. 57+: Black Dragonkin, drakes and whelps south of Blackrock mountain.
Lvl 70+ - Skinned off Coilskar Cobras in Coilskar Point in shadowmoon valley or Twilight Serpents west of Forge Camp Hate in Nagrand. These are used to craft [Cobrahide Leg Armor] and sell about 20g a pop in the Auction House.
Lvl. 62 Elite: Get it by skinning the ancient core hounds in Molten Core, To do so requires you to have a skinning skill of 310, which before The Burning Crusade required you to have [Finkle's Skinner], which is only dropped by The Beast . You can also progress to Zul'Gurub to get the [Zulian Slicer] off High Priest Thekal, which also raises skinning by 10.
Lvl. 42+: All around the Pool of Tears in the Swamp of Sorrows The creatures are Lvl. 42-45. These dragons were once elite, but were recently changed to non-elite.
Lvl. 50+:Temple of Atal'Hakkar (a.k.a. Sunken Temple). There are many elite Green Dragonkin that can drop scales here.
Lvl. 35+:Dustwallow Marsh along the eastern coastline. Mudrock Tortoise range from 36-38.
Lvl. 38-40: Western Badlands near Kargath. Best drop rates from the wolves and cats lvls 38-40.
Lvl. 38-40:Dustwallow Marsh, around Bloodfen Burrow in the southwest (coordinates approx. 31, 65). Raptors have very good drop rate. Good farming spot due to quick respawn and many mobs in a confined area.
Lvl. 25+:Stranglethorn Vale, raptors around Grom'gol Base Camp. Drop rate is VERY high (depending on skinning level).
(these are lower level since Cata but still drop some Heavy and Medium leather you can convert to Heavy leather.)
Lvl. 40+:Stranglethorn Vale along the southern coast line. Jungle Stalker raptors range from 40-43. Great place too, due to the large number of people killing them for quests. These mobs also occasionally drop Thick Leather.
(Since Cata these are replaced with level 27-30 Tigers and Panthers that drop Medium to Heavy leather.)
Across the road to Mistvale Valley are level 31-32 Gorillas that drops Medium to Heavy leather.
Lvl. 40+: Wolves near Aerie Peak in The Hinterlands provide heavy leather and heavy/thick hides. These mobs also occasionally drop Thick Leather.
Lvl. 64+:Nagrand. The Talbuk Thorngrazers around Garadar drop mostly Knothide Leather (65%; about 35% of the time they drop 1-3 scraps) and go down fairly quick, while dealing low damage
Lvl. 61+:Hellfire Peninsula. The Deranged Helboar around Honor Hold , Thrallmar and To the south of Hellfire Citadel do drop Knothide Leather, around 60% of the time they drop 1 maybe 2 scraps and the rest of 40 % 1 Knothide Leather, They Die quite fast, but they do considerable damage.
Lvl. 30-38:Arathi Highlands. Raptors. Highland Strider, Highland Thrasher, and Highland Fleshstalkers. Most roam the zone, the fleshstalkers are found west of Boulderfist Hall.
Lvl 57+ Elite silithids in Silithus. It can also be farmed by killing the Tortured Druids and Sentinels around Southwind Village, who will summon up to three Hive'Ashi Drones upon death. This can be risky and/or lethal, but easier than the slow farming of Elites. (Note: Since patch (1.12?), these undead Elves only summon one drone upon death - which have a fairly high droprate of the Chitins.)
Lvl. 41-44: Crystalvein Mine (42,50) in Stranglethorn Vale (and the immediate surrounding area outside the entrance) has tons of Basilisks that drop Thick Leather most of the time, but also Heavy Leather. Re-spawn rate is excellent, since even at 80, after a full run through the area, re-spawns started showing up.
Lvl. 48+: Southeast to east of Un'Goro Crater. The raptors and the diemetradons range from levels 48-51. Great for farming thick leather!
Lvl. 49-50: Along the eastern shore in The Hinterlands are turtles from level 49-50. They drop thick leather roughly 60% of the time and rugged leather roughly 40% of the time. Be wary of Gammerita, however, an aggressive turtle who will attack on sight. Read a full writeup on the hinterland turtles here.
Lvl. 35+: Northeast coast of Dustwallow Marsh, extending Northwest from Theramore Isle. The turtles range from Lvl. 36-38.
Lvl. 40+: East coast of Tanaris. The turtles along the northern part of the coast are levels 40-43 and usually drop 1 scale at a time. Turtles along the southern coast are levels 48-50 and often drop 1-2 scales. Be mindful of level 50-51 elite giants mixed among the turtles on the southern coast. The southernmost beach in Tanaris, Land's End Beach is loaded with the same 48-50 turtles, and no elite giants. Probably the best and loneliest place to farm Turtle Scales, however, many of these turtles are aggressive (red).
Lvl. 50+: East coast of The Hinterlands. The turtles are level 49-51. These Turtles normally drop 1-3 scales. There is one aggressive named turtle Gammerita that wanders among them who will attack you on sight, so be vigilant when hunting here. Read a full writeup on the hinterland turtles here.
Lvl. 40+:Lethlor Ravine in the eastern Badlands. The whelps there are Lvl. 41-43, and the dragons are 41-50 Elites. The whelps and dragons are separate, so you don't need to worry about getting roasted if you're farming solo.
Lvl. 40+:Sprite Darter in a ravine west of Camp Mojache. The darters here are 42-45 and only killable by members of the Horde.
For Farming locations relevant to wotlk check out Just My Two Copper. Also included are auction house guides for what you can do with all those farmed materials.
If you're having a hard time finding something, and you're willing to spend the money, the Auction Houses are always an option.