Volcarona is a special pokemon that you will only encounter once in Pokemon Black.
After you get the National Pokedex, return to the Relic Castle and you’ll be able to encounter Volcarona near the end of the catacombs.
Volcarona is level 70 when you encounter it and has a low catch rate, so come prepared with high level pokemon and a lot of pokeballs.
Alternatively, you can evolve a Larvesta into a Volcarona at level 59. The only way to get Larvesta is to talk to the person in the house on route 18 to receive a Larvesta egg. You must have room in your party for the egg or you won’t receive it.