Take measurements and go to a frame shop, craft store, etc. They might have the size you need
Are you asking about a bathroom mirror?
Any framer can do any size.
Home Depot and Lows sells corner in their molding section. You then can pick out molding that you like and only need to make straight cuts. I painted mine with Rustoleum spray paint, because I put them on bathroom mirrors and did not want water penetrating the wood. Also spray paint eliminates brush marks. Get a good construction glue and glue these pieces right on top of the mirror. I did it in both my bathrooms and it looks brand new.
If it's for a bathroom you could use casing. Just make sure the mirror is secured to the wall first.