I think that in most cases the mixers and singers are usually friends (or just somehow become friends) or just have connections (for example Singer A is friends with Mixer B and Singer C. A can then get B to mix for C).

I wouldn't be surprised if there was a forum out there with people offering to mix for practice.

Ah, I asked Pomme once, about her mixing in her Karakuri Pierrot cover, she said, there are quite a few people on NND who will accept mixing requests, if I remember right...