Sorry for the delay Grace as I've been on the road a lot (and still am) but definitely nice to meet you. Cool on your new project. It sounds pretty interesting and would love to hear more. Maybe grab a beer next time I hit LA.
As to a music supervisor, I've usually handled a lot of the music for our various films, as when you are doing it low budget DIY it's usually the only way.
With that said, I am familiar and friends with a music supervisor here in SF. Her name is Brooke Wentz (firstname.lastname@example.org) and she runs the companyhttp://www.rightsworkshop.com/. She's just a really good person, great to work with, and has done a lot of indie docs. After we took care of a lot of stuff with Universal (with the band and management's help) we did bring her onto the Fishbone doc to clear some stuff with Sony as she had a good relationship with them and helped get it all worked out in an affordable way. But yeah, she is well connected and good people so probably at least chatting with her and let her know I recommended you all to her.
Also, offhand, one thing I would say to look into when you talk to her, but also just general research is to just try to avoid any classic songs from Detroit, as they are going to be expensive, probably no getting around it no matter how worthy the subject.
There have been so many indie labels in Detroit for decades I think you can find some lost songs that many people haven't heard, but have the sound and style of the times and likely these songs will be cheap and maybe a lot are in the public domain.
I can't think of someone to recommend there right offhand, but someone must have put some compilations together. This is the sort of stuff Brooke can do, as she has the contacts and if you let her know you are looking for songs from the era, but not the hits, then she can probably find them.