The best thing you can do is to have all sources you can get, Soulseek/private trackers/google is often your best chance of getting something you're looking for. As for obscure music trackers to, i guess the obvious that have been mentioned indietorrents and shellife. Shellife having a similar rule to what you are mentioning, no mainstream releases allowed, regardless of label. Both are different sites though no doubt, with a different style of music and in general. If you're looking for things harsh, like punk or noise, go for IT; If however, you are more interested in the lush, pretty side of music shellife has more of it.
As a repository of obscure music, that's the whole idea of p2p, am i right? We all backup some stuff, and we need websites to gather to share. what (or a huge site like that) is the best for that end right now, and it's on the hands of its users to not letting files disappear. All too idealistic i know but it's surprising how big this thing is.