they are near the survival area and resortarea and talk to dawns little sister for areas where there are swarms of Pokemon from the national pokedex
hope this helped you
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The last Pokemon on the national dex is Arceus.
There are 210 in the Sinnoh Dex, 493 in the National Dex {including EVENT Pokemon}
What you have to do is catch all the Pokemon from all the other regions(complete national dex.
In Pokemon Platinum to get the National Dex you will need to have seen all 210 Pokemon in the Sinnoh Dex then talk to Professor Rowan and then after a while Professor Oak will give you the National Dex!
you compete the sinnoh dex.