Where to find Unknown?

Unown isn't available yet.

Wait, there's unown in the game?? Or is it just rumors??

If they took cues from the main series, then maybe near museums.

Unown is available. I caught one in NYC at a random non park or water building.

Nah, there is Unown out there, I have found 1, but it wasnt like a specific biome or anything, just between living houses, couple of pidgeys and swinubs around it.
When I saw it I thought hes not gonna be that rare, I think it was like Day 2 of gen 2, but I must have been super lucky