the only way is if you fight Pokemon trainers like, collectors or scientists. to be a little more specific, there is a Pokemon collector on route 214 who has a porygon.
you cant find a porygon ,you can find it in Pokemon pearl or on platinum
For Porygon to evolve you have to trade it whilst it is holding an 'upgrade'. See related links for more info.
An Up-Grade is an item that evolves the Pokemon Porygon when Porygon is traded while holding it. Porygon evolves into Porygon 2.
i think you see it on a route near snowpoint city its covered with snow
Porygon
you can get porygon in platinum by going to velistone first, and then go to the top house on the top of the Pokemon center.Finally,you ask the man inside the house, then he will give you a porygon level 25.
you have to link trade porygon
Porygon
you must trad a porygon holding an up-grade
find porgon 1 and catch it then in the closest bit of grass you will see porgon 2
Go to Vielstone City with a spare spot in your party and go to one of the houses behind the Poke centre. A man will give you Porygon.
Pokemon 192 or Porygon,can be found in Veilstone city go behind the Pokemon center you will see two houses go to the one on the right and you will see and old man talk to him an he will give you a Porygon(make sure you have open party space)