I really want to finish my Johto Pokedex in Pokemon Go but cannot find an Unown and have no idea how to get one.
(If you have one now then this is sort of irrelevant, so if so just carry on with your day)
As a bit of extra info to add to Jhnfui's answer , typically they appear during special events, so something like Go Fest.
Asides from that, you just have to get lucky.
Unfortunately, Unown is a very rare pokémon in Pokémon Go. By rare, I mean very rare. As a side note, there is no specific place it can be at because it is a Psychic type. so you will just need a lot of good luck.