SoundBetter is a great site to find freelancers in the audio world, including mixing engineers. Here you can talk to the mixing engineers to ask any questions you have, and you can listen to their previous work. Other freelance sites, such as Fiverr and Upwork, are also great for finding freelance mixing engineers, even though these sites are for freelancers of all varieties. Another option is to look at songs you love the mix for, then find out who mixed them. Depending on your budget, you could get your music mixed by someone who mixed your favorite song.