Basically all of the mid-30s zones have a significant amount. Alterac Mountains have some if you go to the snowy area. Arathi has some if you look for the rocky places. Stranglethorn Vale has all the mid-level herbs you could ever handle. All of these are profitable but, on PvP servers, dangerous.
If you're looking for quieter spots, I'd recommend Badlands and Swamp of Sorrows. Badlands supplements the Goldthorn with Kingsblood and Firebloom and the occasional Khadgar's Whisker. Swamp of Sorrows has Goldthorn along the ridges and absolutely copious Blindweed, Liferoot and Kingsblood in the wetter parts.