Another geeky fun resource: The Plainsong and Medieval Music Society. Academic. But seem to specialize in publishing obscure hymns and songs, such as a sequence for a Mass for St. Ursula (and the 11,000 virgins) in the old York Rite.
I also must add it’s so moving to come across versions of still-sung hymns in ancient manuscripts and hear more or less the exact same chant I know come off the page. Here’s a transcription of a version of Vexilla Regis (which friends and I sang at vespers of Palm Sunday) from a manuscript from 16th century Toledo, Spain:
Do be generous and donate to support projects like this if you download or use material from the site! Here’s their donations page.