You can't buy Phoenix Downs ever in the game. Their considered to be a rare item on the level of Elixers.

But as far as reviving goes, every town has a building with a revive spring in it. In Ur it's in the elder's house. In Kazus it's the first building as you walk into town. Just walk into any unmarked building and you'll eventually find it. Though some places don't have buildings, then in which case it's probably just some pool of water somewhere like in Castle Argus.