You believe everything you find on Google? Black and White doesn't have any pokemon from outsite Unova until after the Elite Four. It isn't like other games where you have pokemon from previous generations mixed in with current generation pokemon. Fortunately you only have to get as far as Driftveil to get the National Pokedex from Juniper's father. That or Mistralton. Still, even after that none of the trainers use non-Unova Pokemon until after the Elite Four, the final battle with N, and Ghetsis. That said, a trade after the dex but before those battles may or may not work. Either way, I'm not inclined to lend my Ditto. Sorry.