Where do you get Porygon in Pokemon Platinum?

In Pokemon Platinum (I don't know about Diamond and Pearl), Porygon is located in a house directly behind the Pokemon Center. If you want to evolve into Porygon 2, trade it holding a Up-Grade, and if you want to evolve Porygon 2 into Porygon Z, trade it holding a Dubious Disk. I do not know where htese items are found. If you want to get Porygon the harder way, use Pokecheck.