I'm gonna second Soulseek. It is pretty old, but so many of the people who use it are collectors who organize their collections well, and I've found some pretty obscure stuff on there as well.
I haven't tried it myself, but I want to try Soularr, which apparently lets Lidarr use Soulseek as a source.
Man, that takes me back. Several gaming groups I was part of had Hotline servers. The role they served for us was probably closer to a Discord server.