I really want to finish my Johto Pokedex in Pokemon Go but cannot find an Unown and have no idea how to get one.
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.
(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.