Actually Mightylink, you're able to obtain nearly the 493 Pokemon in Diamond/Pearl/Platinum without the need for migration (With exceptions of few, hence nearly).
After you beat the Elite Four and obtain the National Dex, there would be new Pokemon lying around that come up (Chances being higher) with the 'Pokeradar'. They're littered throughout Sinnoh. Some special Pokemon appear in the Trophy Garden during days of the week as well. Don't forget about the island in the back where there are special National Dex Pokemon.
Finally, in Sandgem Town, every day there will be a special influx of a certain Pokemon at that route.
You can get all the Pokemon except for version exclusions, legendaries from previous generations, and starters, if I recall correctly.