in platinum go to snowpiont city there will be a clown to your right [beside the Pokemon center]
good,luck
A trainer on iron island has hippotas
Have you seen a trainer with one? if so go to your pokedex search for the hippotas and go to "Area"
hippotas is on iron island
In the elite 4
in the Maniac Tunnel can you find them
the land trainer in the elite four Pokemon league castle
uxie,hippotas,hippodon and snorlax(yes he is in platinum).
You can ask friend for a hipodon mate it with a ditto
route 214 it may be rare catch
You find hippopotas (note I spelled it right) in the maniac dude's cave
There is no trainer in platinum that is willing to trade a munchlax.
Sorry but there isn't a trainer with a bagon in platinum.