Behold, once again, an easily answered question can be found in the Trade Skills Sticky right at the top of this page!
Quote:
Q. I hit 150 skill, and there's no trainer! What do I do!?
A. You need to buy the First Aid manual. For Alliance, the vendor (Deneb Walker, I think) is in the ruined city of Stromgarde, in Arathi Highlands. He is in the Alliance-friendly section, right outside the prince's room. For the Horde, the vendor is Ghok'kah, in Dustwallow Marsh.
Note: These vendors also sell the books for Heavy Silk and Mageweave bandages. Buy them!!
Alternatively, look on the left for the "List fo Trainers: link to show you where to go, including links.