you dig them up in the under ground mine
buy the Pokemon platinum book
you find them in the cave
spell you trash 'platnium'=Platinum
Find all plates and got to spear pillar................. TRICKED YOU....you need the Azure flute or action replay.
In the underground, they'll appear all of them, randomly.
you see all the Pokemon in platinum and trade from your other games like emerald, blue, fire red etc and that is how you find all Pokemon in platinum.
You can find a bunch of Pokemon but i realy don't want to list them.
Give them to your Arceus, if you have one, or a Pokemon to boost a specific move's type, Or you can sell them for 500 each.
You can find a Hippopotas in Pokemon platinum at "Ruin Maniacs Cave". If you need to find something else about Pokemon go to Serebii.net. They have all the information about Pokemon! I hope I helped!!
you can find them in gartinas cave
you need to download your Pokemon platinum game into the R4 and add cheats in there . Go to Supercheat to find the cheats
You have to defeat all the trainers and find all the Pokemon