A good cloak for Fire Resistance is the Onyxia Scale Cloak (+16 FR) if you add a Greater Fire Resistance Enchant you get +31 FR (16 + 15 from enchant). At level 67 the vendor-obtainable Wyrmcultist's Cloak (+24 FR) becomes available, for a total of +39 FR with enchant.
A good example of a plate FR gear set for a Illidan off tank would be as follows. All Inferno Tempered FR gear (200 FR) bought with 100 badges. Both Phoenix-fire Band (30 FR) & Pendant of Frozen Flame (30 FR). Equipping The Fel Barrier (22 FR) and enchanting Enchant Cloak - Greater Fire Resistance (15 FR) on a epic quality tanking cloak that probably isn't used anymore like Gilded Thorium Cloak. This totals to 297 FR unbuffed. When buffed with either +70 FR from a Paladin's fire aura or shaman's fire resistance totem, you will comfortably arrive at 367 FR (365 FR is the cap, anything over is wasted). This gear set is recommended because it allows using almost completely epic gear, generally carrying the most stamina, when using as few pieces as possible. As the adds during the Illidan fight can crit, using as few pieces of FR gear allows the rest of the gear to make up the much needed 490 defense skill needed to be crit immune.