I note that my rocket ammo does not respawn with Gunzerking, and I rarely find any in crates and such. I've checked the ammo vending machine and it does not have any. Is it possible to buy rockets, or are they found only? Where is a good place to find them.
You can buy Launcher ammo but not in the first few areas' Ammo Dumps; by the time you reach Sanctuary you should find them; I know for a fact Sanctuary eventually stocks them for the rest of the game, but it might be level-restricted.
Rocket Launchers and several other guns (elemental guns, SMGs) seem to be restricted to mid-game levels only. You were "lucky" to find them before being able to get ammo for them, but rest assured it's just as easy to get ammo for launchers as it is other guns later on.