I'm sorry, but it cannot be found in Pokemon White or Black. It can only be obtained through trading. :(
However, you can get it in Black/White 2 by climbing up the stairs in the Castelia Sewers. There will be tall grass, and there is a 5% chance to encounter one. You also get one from Amanita in Castelia City.
Sorry for the bad news, but I hope I helped! :) Source