The page here says you can find it at the Thrifty Market, but I can't find any grass or anything like that. Is it moon exclusive? Do you need to beat a certain person? I'm on my way to the fifth trial, just fyi.
You can find Mimikyu at the Thrifty Market just by walking around. It's not a version exclusive, so you can find it in both Pokémon Sun and Moon. It might take a while though, Mimikyu has only a 5% encounter rate!
Hope I helped!