Finishing Sinnoh Dex

First off, you do not need to have seen Manaphy to get the National PokeDex. Secondly, Beat the Elite Four and you will see Vespiquen, Garchomp, Snorlax, Hippowdon, Drapion, and Milotic. For the remaining Pokemon just go and battle every trainer you can find.

Goo goo g'joob

Walrus guy is right, but if you really want to get Manaphy for your pokedex, (even though you don't need it) You would have to have the Pokemon Ranger game. When you complete that game you can get a manaphy and transfer it to your Pearl game. Just so you know in case. :]

Ok here is my advice vespiquen,snorlax milotic,Garchomp Hippowdon Drapion
Can be found in the Pokemon league however I cannotrecall if the rest that you are looking for are in victtory road. But for clefable go outside solaceon city towards celestic and veilstone and you should see a pair of trainers one will have a raichu and a clefable. Manaphy is an extra and you don't need it to finish the pokedex yet. For Mantyne raise a mantyke with a remoraid in your party. For Luminion either fish for it witha super rod in canalave city or battle Good luck

No trainer has a Manphy so get it by trade, Garchomp is from a Dragon trainer, Vispqeen is in the Elite four, Miltoc is from the battle tower