I'm currently working on an app which would help musicians collaborate schedule with each other easier then via email/phone etc.. Also for groups like Bands, Choirs etc.. But I'm having a hard time finding the right research about how many serious musicians are out there doing it for a living, Where would you look as a start? I've tried searching for "band forums" "musician forums" etc.. But that doesn't help much
I would suggest spending some money and cash hanging out at bars where some of these bands perform, Also, try to find a way backstage to talk to the musicians after they perform to do your research. If you're going after well-off musicians, contact their agents or band managers. They are always looking to make or save more money.
As a long time musician and performer, I can understand the issue here. The sad fact is, today - many musicians still rely on the literal "bulletin board" for finding new band members, promoting performances, etc. Or, they create printed and digital flyers and try to grow their own community via Facebook, Bandcamp etc.
Today, I think the best way would be some targeted ads on places like Facebook, Bandcamp, Soundcloud, Monkeybars and Grooveshark - if budget exists.
Otherwise, post heavily on as many musical equipment message boards and forums, Craigslist etc. as you can physically manage - all for free and see what kind of critical mass you can accumulate.