well there is what.cd but it requires an interview to get an account for. and since i don't have an account i really don't know exactly the extent of rare music they have on there.
aside from that, unfortunately in my experience, for rare music it's either crafty search engine use or buying/importing the physical.
there are/used to be a lot of blogspot blogs which would post rare music downloads so sometimes adding the term "blogspot" to your search can help you. there is a wide variety of torrent tracker sites of course but whether they will have what you're looking for is anybody's guess. i have gotten lucky before and hit the jackpot with entire discographies of rare artists. if you want to buy the physicals i'd recommend Discogs.com, it's a database of physical music releases with an integrated eBay-style marketplace.
and if what you're looking for is on YouTube then just use a downloader lol. quality can be kind of random tho.