Hmmm I dunno to be honest! They're probably friends with some of the mixers or ask them for help, sorta like how producers or singers requests artists for their art. I don't know if there's a particular board (like on bbs boards) for it, or people just ask on Twitter/skype.

Some singers just mix themselves but there are some famous ones too (like Pupi, madamxx, and er... I don't know many orz)