Here is a link to the CyberBass study files for Vivaldi’s Gloria.
Here is a link to the CyberBass study files for Rossini’s Petite Messe solennelle.
Here is another resource called Chord Perfect, which may offer a better solution than CyberBass. They use actual singers rather than digitized vocal sounds, and the video shows the choral score. If you want to download the stuff it costs $10, but they’ve posted the files YouTube for streaming. Note: there is a bit of divisi in this work, so they’ve provided separate links for the Soprano, Alto, and Tenor parts.
Here are links to Chord Perfect’s playlists on YouTube.
Finally, here is an excellent performance, using the version we will sing. The chorus uses boys for the soprano part.
02. Gloria in excelsis
03. Gratias agimus tibi
04. Domine Deus
05. Qui tollis
07. Cum Sancto Spiritu
08. Credo in unum Deum
10. Et resurrexit
12. Agnus Dei