Have you looked into getting books which are for voice and piano, but can just play the vocal line on the piano so that it can be used as an instrumental? Some arrangements will also include the vocal lines in the accompaniments.
I have the following books that you might want to look up, which I think some of the pieces can be used as instrumentals. I looked at a few and some had the vocal lines already in the accompaniments. Others, you would just have to include yourself.
My repertoire is more classically based as that is what I'm trained in doing, so keep that in mind. I don't really have much in repertoire for contemporary music, but I think some of them would sound very nice on piano.
- "Ave Maria - Vocal Album with settings from the 16th to the 20th century for 1-4 Voices and Piano" - Publisher: Schott Edited by Lydia Abramova (The title might also be in German "Vokalalbum mit Vertonungen aus dem 16. bis 20. Jahrhundert fur 1-4 Singstimmen und Klavier")
This has a boat load of different settings to the "Ave Maria". Just be aware that some of the pieces in the book, although religious in content, may not be appropriate for mass, especially four of the pieces which are from operas which are designated in the table of contents. (Technically shouldn't be used as they are from secular works)
- "Ave Maria and Other Great Sacred Solos - 41 songs for Voice and Keyboard" Publisher: Dover Publications Edited by Rollin Smith
This one includes a Topical Index for every piece in the book. Ie. Adoration and Praise, Christmas, Communion, Consolation,, Easter, General, Lent and Holy Week, New Year, Palm Sunday, Prayer and Supplication and Virgin Mary
- "Soloists Practical Library of Sacred Songs" - Collected and Edited by Richard D. Row Publisher: Carl Fischer
Hope it helps.