Does that sound strange?
Don’t worry, here is your step by step guide to finding music for a simple Sung Mass according to the Extraordinary Form of the Roman Rite.
I make up folders with music in the order in which we will sing it. This can be tedious to prepare, but saves a lot of hassle in rehearsal and during Mass. Sometimes I also prepare CDs for singers without internet access. Today I found this handy page for making CD pockets with track listings.
Prayer before choir practice: from my own choir page repleatur.net. Not strictly necessary, but a nice way to start. This isn’t just any old rehearsal, this is Prayer.
Asperges or Vidi aquam for the Rite of Sprinkling: I prefer the version from the Institute of Christ the King Sovereign Priest as it includes the prayers following the chant: find them here.
Kyrie: This is from the Kyriale. Antoine Daniel Kyriale is a good source. There are 18 Gregorian chant settings, plus extra ad libitum versions, with links to certain feasts and seasons, but you are not strictly bound to adhere to those recommendations.
Gloria: Also from the Kyriale (see above)
Gradual and Alleluia: I find these the trickiest and tend to go for the psalm tones here. They come under the Propers, see Introit.
Credo: This is from the Kyriale, but is not part of the 18 Gregorian Mass settings. There are 6 Credo settings, usually listed after the 18 Masses. Credo 3 is good with Mass 8. Credo 1 is the default setting.
Offertory: This comes from the Propers, see Introit.
(you might like to fit a hymn or motet in here, depending on time)
Sanctus: This comes from the Kyriale.
Agnus Dei: Another one from the Kyriale.
Communion: Another one from the Propers, see Introit.
(another chance for an extra hymn or motet)
Ite Missa Est: The Institute of Christ the King includes this in their Mass settings. Some Kyriales leave them out as they are quite short and usually much the same as the tune for the Kyrie, but good to have a copy on hand to cover all bases. The priest may expect you to give the first few notes. It is very good to check exactly who will be doing what before Mass.
Then there is often a Recessional Hymn.
A great resource is Psallite Sapienter: A Musician’s Guide to the 1962 Missal, available to download from Musica Sacra Book PDFs or in print. There is some variation in applying these guidelines, this book tends to take the safer road wherever there is any possibility of other interpretations.
For a handy source of extra music you might like to browse A New Book of Old Hymns, especially set up for singers who like to see an English translation. It includes music for all seasons, focussing on Gregorian hymns.