There's only one way to get a Porygon in Hg/Ss. And that is...the Game Corner!
Sorry, but that's the ONLY way to get a Porygon. Here are some alternatives.
- Trade one from Diamond or Pearl (Trophy Garden)
- Trade one from Platinum (You get it in Veilstone City as a Gift Pokemon)
- Migrate one from FireRed or LeafGreen (You get it in the Rocket Game Corner)