This resource comes out of a Showcase rehearsal with Connie. Besides being an asset of talent to our organization - the woman is a wealth of knowledge in terms of material. We got onto the subject of tracking down unusual and hard to find pieces of music that fit what we do. We have musicnotes et al which are certainly convenient. And our public library has some reserves which are mostly on hold. But she had recommended Bagaduce Music.
It sounds like it works as a co-op library based in Maine of all places. You buy an annual membership which then gives you access to their files. After that you pay for processing which must include mailing rates per song and there are limits as to how much you can have "checked out" during any period. They tout having over 250,000 pieces of rare and hard to find, including choral works as well. I did not spend a lot of time searching but it sounds like a great resource when we are hitting dead ends locally. Give it a spin - no charge to search, only when borrowing.