As one of the best bug-type pokemon available - where else will you find a bug that can use Fly? - Volcarona is in high demand among Pokemon Black and White 2 players. There are few bug-types which can serve so effectively as sweepers, especially considering the game's collection of Elite Four. So how to you snag a Volcarona?
Doing so is essentially the same as it was in Pokemon Black and White, though you have less options. The single Volcarona in these games is found at the bottom of the Relic Castle, just as it was in Black and White. This time, however, you'll have to go through the Relic Passage to get to it. You can do this by travelling through the sewers in Castelia City. Search long enough and you'll find the lower part of the Relic Castle ruins, buried under the Desert Resort. A Volcarona is waiting within.
This leaves one more question open: how do you get Volcarona's pre-evolution, Larvesta? There are only two ways: trade or breed. Larvesta is given to you by a trainer in Black and White, and you can trade that pokemon over to fill up your Pokedex. Lacking that, you'll have to breed Volcarona with another pokemon. Make sure you fight a female Volcarona if you want to get a Larvesta.