The NPC that teaches you this, and all other higher level trainer taught plans, is in Booty Bay. He is at the very top level somewhat near the tunnel out to Stranglethorn.
If I was in a bad mood and felt like being mean I could point out that doing a search on the plans in the Allakhazam database would have told you who sold it.
Also could have just looked up a list of trainers, conveintently located in the left hand menu bar, and found the one in Booty Bay.
Course, I'm not in a bad mood and don't want to be mean.