There isn't an in-game trade to my knowledge, however Hippowdon will swarm at the Desert Resort (after you beat the game).
To check swarms, go through a gate between/ connecting two cities (e.g. Castelia City and Sky Arrow Bridge), stand on the Information mat, click A, and it will show you the scrolling text that says "There's an outbreak of Pokemon at xxx!" If it's Desert Resort, it'll be Hippowdon.