You find hippopotas (note I spelled it right) in the maniac dude's cave
uxie,hippotas,hippodon and snorlax(yes he is in platinum).
You can ask friend for a hipodon mate it with a ditto
Have you seen a trainer with one? if so go to your pokedex search for the hippotas and go to "Area"
route 214 it may be rare catch
A trainer on iron island has hippotas
in platinum go to snowpiont city there will be a clown to your right [beside the Pokemon center] good,luck
in the Maniac Tunnel can you find them
hmm, hippopotas i think u can get from maniac ruins on route 214 but u need to catch every Unown
They are in the maniac tunnel.
you see it in vellstone city
the land trainer in the elite four Pokemon league castle
on platinum, you see Manaphy in Mr.backlots Pokemon mansion