Just go to Pokemon platinum central and click on pokedex and it shows you where to battle all the Pokemon in sinnoh.
You have to find all of the Pokemon in Sinnoh.
this question is worded weird but if your question is "where do you get all thePokemon in Pokemon platinum?" then the answer is to battle every trainer in Sinnoh.
find all 210 Pokemon on sinnoh pokedex then visit profeser rowan
first you have to finish the sinnoh pokedex.then you show your work to professer rowan
you have to see all of the Pokemon and then you get the national pokedex from Prof. Birch and Prof. Oak in Prof. Birch's lab after completeing the Sinnoh pokedex.
You can catch them on Iron Island once you have the National Pokedex. You get this by seeing all the Pokemon on the Sinnoh Pokedex.
first you have to battle with every pokemon like i did! it took me a long time but i did it on platinum i also have all legendary pokemon
get all the Pokemon in your pokedex from sinnoh then talk to professor rowan and oak will give it to ya
once you have seen all the Pokemon in the Sinnoh region go to Professor Rowan and he will give it to you
They have changed it in the newer games so that all you have to do is SEE all the Pokemon instead of CATCHING all of the Pokemon
You get the sinnoh one basically after starting the game, the you get the national pokedex when you´ve completed the sinnoh pokedex (by seeing all the Pokemon in sinnoh)