In the Porygon Pokedex, it says that Porygon isn't in Platium. But according to Bulbepedia Pokemon.Marriland, and Serrebii, Porygon is in Platium.
I think you get him from a guy in Veilstone city because he says he wanted to defeat team galactic or something. He is right by the team galactic base in a house. He is either level 35 or 25.
HTH =P
