Mismagius in Pokemon GO can only be evolved using a Sinnoh Stone, one of two regional evolution stones found in game. Sinnoh stones are used to evolve Pokemon that received an additional evolutionary form in Generation 4. These Pokemon are Rhyperior, Tangrowth, Lickilicky, Ambipom, Yanmega, Gallade, Frosslass, Electivire, Magmortar, Togekiss, Honchkrow, Gliscor, Weavile, Roserade, Porygon Z, Dusnoir, Mamoswine, and Mismagius.
To get a Mismagius, first you'll need to catch a Misdreavus. Ghost-type Pokemon are more commonly found in foggy weather as well as at night. Once caught you'll need 100 Misdreavus Candies as well as a Sinnoh Stone to evolve it into a Mismagius.
Mismagius Pokemon GO: How to Catch
There are a couple of ways to get a Sinnoh Stone. First, you can get one as a tier two reward for completing Mega Battle Challenge research tasks. This is a one time deal though so if you want more than one stone you'll have to rely on the following methods.
Sinnoh Stones can also be rewarded for 7th day field research. They can also be rewarded for defeating Team GO Rocket Leaders or as rewards for GO League Battles. All of these methods are random though, so you might receive a Unova Stone or other unneeded item instead.