Dragon's Teeth can only be found in the Badlands on Eastern Kingdoms, specifically in the Scar of the Worldbreaker. A skill level of 195 in Herbalism is required to gather this herb. It's a grind to harvest, but the rewards are plentiful. Dragon's Teeth can be a lucrative selling item in the Auction House because of its rarity and somewhat painstaking and time consuming collection-effort.
A simple harvesting tactic is to simply travel up and down the Scar repeatedly. However a large, respawning fire elemental also travels up and down the Scar, complicating the process.