You will have to evolve a Porygon. By evolving it you have to trade it while it is holding an Up-Grade.
That Would be Either Porygon or Porygon-2
You must evolve Porygon (From Celadon) using the UpGrade to Porygon 2.
it evolves porygon into prygon z and the up grade evolves it into porygon 2
You cannot catch Porygon2 in the wild, you will have to trade a Porygon holding an Up-grade to evolve it into Porygon2.
Trade it while it is holding a Dubious Disc.
give it to porygon and trade to get porygon-2
give porygon (or possibly porygon 2) dubious disc ( i think you nick it from galactic hq) and trade him.
The dubious disc is to be used with a Porygon 2. First, you must have obtained the porygon. To get a porygon 2, you trade a porygon to someone with it holding an upgrade from Silph Co., and then trade it back again with the dubious disc to get Porygon Z.
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.
there is porygon-2 but not porygon. porygon-2 is in oil field hideout
This is for pearl i dont know if it is the same. Talk to the owner of the trophy garden Mr Backlot and say yes to everything he says he with tell you there isa porygon in the garden use the pokeradar to find it Then talk to oak in eterna and he will give you an up Grade give it to porygon and trade it and it will evolve into porygon 2 Then go to rout 225 and look for the dubious disc and give it to porygon 2 and trade it and it will evolve into Porygon Z
It is Impossiple to catch Porygon 2 in Pokemon soul silver, and heart gold. The only way to obtain Porygon 2 in Soul Silver, and Heart Gold, is to trade a Porygon holding the Up-Grade with a friend, or with yourself. You can find the Up-Grade at the Silpho Co. Building by talking to the people on the first floor.