Wow - check this out! You can actually enter some (at least 3, it says) notes of any chant melody, and the database will return all pieces of melodies that contain that sequence! And then link you to square note chant scores of each item returned!
It also notes that "The pitch of the first note is not important - in the search results the melody appears in all transpositions."
Amazing! The website is called "Global Chant Database," and I haven't looked at anything but the Search feature yet, so amazed am I at that.
This is just terrific!