I met this girl who said she would trade Hippowdon for Alakazam, but I have no idea where to find Hippowdon! Where can you find it?
Hippowdon can be found in the Desert Resort during swarms.
Hippowdon (Bulbapedia)
Unfortunately there is no way to catch Hippowdon in the wild in Black/White 2, but it is possible to catch a Hippopotas on route 4 in Black and White, during a swarm. You can then evolve it in to a Hippowdon at level 34.