They are rare. Pokemesh can help.
Pokemesh is a tracker that uses GPS hacking to give you spawn points with timers telling you when the pokemon will despawn. Unlike other services that got banned for using the same fake accounts for tens of thousands of users, for pokemesh you need to make your own secondary google and pokemon go account. Never run it on your main account.
So you have one google + pokemon go account that you play, and a second one for pokemesh, and the second one stays level 1 because it does not activate pokestops and does not catch pokemon, it just looks around when you activate pokemesh. It only looks at nearby areas and superimposes the data on a google maps map. That secondary account could get banned, but your main account should be safe.
The old trackers that no longer work used a few fake accounts for thousands of users, causing the fake accounts to generate a lot of traffic and teleporting across nations and continents. Pokemesh uses one fake account per user that you need to create yourself, and only when you decide to use pokemesh, that fake account will slowly check out the areas around you, walking distance, one location at a time, forming a grid.
The idea is that its only lvl1 fake accounts that are not active 24/7 but only when one specific user uses pokemesh, and the movement is not completely whacky but in line with a bike user.
Porygon is very, very rare, i do not think that there is a specific area. I only had one in my nearby-list once, which caused 2 other players to get on their bikes and canvas the nearby areas, with no success.
