You can find propers for an enormous number of saints' days in the various volumes of Paleographie Musicale, which is in every music library worth its salt (but of course the number of music libraries is limited unless you are in a more or less cultural center). Anyway, I have used these volumes many times to find chant that is impossible to find in a modern edition. The trick is transcribing it - these are facsimiles of manuscripts, so you need to get it a copy to work on far enough in advance so that you can get it done in time for the choir to practice. Notation in some of the earlier manuscripts is beyond my abilities or is in ligatures that only show relative pitch, but in general you can find one in a notation you can read. It's definitely the first place I go for unusual chants, either Mass or Office.