#$@&#$& spell correct! From my Kindle.

Found it.
https://www.baerenreiter.com/en/sheetmusic/product/?artNo=BA5701

I'd hate to hear what the people who are too cheap to buy the Liber Hymnarius will say about the price tag on this one...

An index of Hymn melodies can be found here, it contains in it's indices the Stablein.

http://www.globalchant.org/index.php

Please note this is not complete (it does not have the Dominican books etc.)

Also the incipit classification can not differenciate between different variations of a melody.