Ummm.. you find them?!?!?!?!? I'm sorry but that is probably the most dumbest Pokemon question I've ever answered! You look for them and trade with other trainers. Or, if you're like the cheaters, us an ar.
Skill and a lot of pokeballs.
To finish the sinnoh dex (Pokemon platinum's,diamond, and pearl dex) you just need to see every Pokemon not catch every Pokemon.
battle trainers or catch the Pokemon
depending on where you go and what you do you can catch all of them, every Pokemon in existence
how do you catch a mewtwo on Pokemon platinum
bet every gym but the last
you cannot catch the 3 regi's in Pokemon platinum you have to catch them in Pokemon emerald then transfer them to Pokemon platinum. thnx
when you catch all the unkown in platinum included every letter
Catch Alot Of Diffrent Pokemon,Pokemon Global Trade Center,Migrate, And Just Try Really Hard.
you can catch one every Friday at the valley windworks
You can't actually catch him in Pokemon platinum but you can migrate him over to platinum if you have him in another game
you can't catch him in platinum but in soulsilver or heartgold you can catch him then trade it to platinum.
try it again and if that doesnt work you need to catch every Pokemon