You should see them all during the course of your game - make sure you battle every trainer!
If not, you can find them here:
- Drifloon is outside the Valley Windworks on Fridays.
- Ambipom you have to evolve Aipom, who you get from putting honey on trees.
- Lopunny you have to evolve Buneary
- Whiscash is fairly common from fishing
- Hippopotas is in the ruin maniac's cave