If he's looking for sources to make his own Chant Chintz, Google Images is his friend, though reproduction rights would need to be considered:

https://www.google.com/search?q=chant+manuscripts&sxsrf=ALeKk032lS3XX7Ba3BsRIv74GxUKN9pr4g:1596723708521&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjtpM2r44brAhXhmnIEHcANBnYQ_AUoAnoECA0QBA&biw=2202&bih=1040

There are numerous reproductions of mediaeval manuscripts in La notation musicale de chants liturgiques latins, a Solesmes publication which may be had from GIA. Whether there would be a copy right problem in your situation I don't know. I have made copies of many of them for use in illustrating lectures. Since you are using them for classroom use I think that that would be alright.

This is a very good book for any chant enthusiast or scholar.

on the CMAA site, under the tab "Book PDFs", you will find "Music Fourth Year, by Justine Ward (1921)". This has at least 12 pages of illustrations which could be enlarged and framed - in fact, if I ever get my choir room, I hope to do this.

On page 2 is a drawing of Mater Ecclesia, showing people of all ranks and kinds sheltering under her outspread mantle which wraps them in Gregorian chant. :-)

Mmeladirectress

That picture of our lady garbed in chant is really cool. I believe it is called the patrocinium.