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give porygon (or possibly porygon 2) dubious disc ( i think you nick it from galactic hq) and trade him.
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.
Get a porygon 2 make it hold the upgrade and then trade. -Hope I Helped!
You can give it to a Porygon and trade it so it will evolve into Porygon2.
Have one space available in your party then go to veilstone city go to the house on top of the Pokemon ceneter go in and talk to the person and you will get a lvl 25 porygon