The Music section of the HBLL has an extensive collection of scores suitable for choirs or special musical numbers. Find them by searching "Mormon Music Collection" in a general keyword search.
Basic range of call numbers for sacred choral music :
M 2020 – M 2101.5
M 2020 - M 2036 = accompaniment of orchestra or instrumental ensemble
M 2061 – M 2080 = accompaniment of keyboard instrument
M 2081 – 2100 = unaccompanied
M 2101.5 = unison choruses
Compositions by LDS composers are designated by a .8 after the call number (so, for example: M 2061.8, or M 2101.58).
Scores held in the library lack sufficient number of copies for a choir; some titles have enough copies for a smaller ensemble. If you find something you would like to use with a choir, library staff can help you discover where more copies are available.
Betsy Lee Bailey notes that it can be difficult to keep a ward choir going all year. Why not switch things up, and have a family choir? See Sister Bailey's ideas and PDFs of suggested music for this purpose here.