You should be able to get them all in your pokedex straight after you defeat the champion. If you are missing a couple, make sure you've battled everyone along the way and make sure you have visited the three lake pokemon, at lake valor, acuity and verity.
It is possible to get every pokemon from Platinum by wild encounters, or seeing them in a trainer battle. By the time you defeat the champion, the pokedex should be complete. Usually, however, it wont be because maybe you didnt battle everyone, or something. In that case, do one of the following: 1) go back and look for the trainers you may have missed, 2)train at the battle frontier. You are likely to see different pokemon than just that of the Sinnoh region 3) trade from Pokemon diamond or pearl 4)trade from fire red, leaf green, etc, through pal park or 5) use a gameshark code.
You would have to migrate pokemon to catch them all. but to get all the data in the sinnoh dex you just need to battle every trainer along the way to the elite four and then afterwards go battle/ catch Rotom in the old chateau in eterna forest.
