I am an organ/piano musician for a Catholic church with little control over the music sung by our choir, most of which is not what I believe appropriate for a traditional Catholic Mass. However, for the music not involving the choir, I make my own selections. Unfortunately, my collection of traditional sacred music is extremely limited. I have had a very difficult time trying to find a collection of such music (for keyboard playing). Any information that anyone may offer in acquiring such music would be deeply appreciated.
Having identified the chants specified for the day, look for pieces based on those chant melodies, or improvise on those chant melodies, or peruse the "Nova Organi Harmonia" (NOH) for an organ accompaniment to the above chants. http://www.jeandelalande.org click the full-page icon scroll down and click ADDITIONAL TOME (Organ Accompaniments) The NOH has eight volumes, but only seven have been found and scanned. Each PDF is 29-37 MB so right-click and save them to your hard-drive.
At the top of any Forum webpage click the blue word Search in the search box enter organ click radio button Topic and click button Search, or click radio button Comments and click button Search.
Don't overlook the repertoire of Spanish and Portuguese organ music from the 16th and 17th centuries. Much of it is manuals only; some of it is harmonically pungent in a way you don't expect from other countries' sacred works; and all of it makes an agreeable change from later and better known areas of organ history.
Here are some examples (non-copyright, I gather) that I found after a quick trawl through the Net:
(a) A Fabourdon by Antonio de Cabezon (Spain, 1510?-1566)
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