There's a similar thread here;
https://www.gearslutz.com/board/mastering-forum/37418-why-can-you-attend-mastering-session-but-not-mixing.html
The best way to find a good mix engineer is to start hunting around and gather some names. Hunt down their work to see if you like the mixes and see if they translate and sound good in a variety of different environments, I mean...that IS one of the technical points of mixing. A good mix will travel from headphones to a $50K stereo and everything in-between without falling apart.
Beyond that, talk to the engineers and get a feel on them and if they “get” your music. You probably don't want to hire the big name metal or classical guy if your doing jazz, but on the flip side...a good engineer will be (or should be) able to understand a lot of different genre's and, provided the tracks were recorded decently, they'll be able to put together a solid mix. With digital tracks, FTP sites, and high-bandwidth internet connections distance doesn’t matter like it did five years ago.
Rates can really vary, and the highest day rate isn’t always the most expensive option for the project. It depends on how many mixes can get knocked off in a day, typically two or at best three is a good number but depending on the complexity of the song and tracking, it could go faster or slower.