Here are some links to classical music libraries and other useful sites. You can download music in various formats including pdf and audio.
Choralia provides free training aids for choral singers.
In addition to its own extensive collection, the Icking Music Archive site also has links to many other free music libraries.
Eyes and Ears An anthology of melodies for sight singing, by Benjamin Crowell.
Roni Music provides the Amazing Slow Downer, which slows down music from CDs or audio files without changing the pitch. This is really good for learning fast moving choral works.
Note Perfect provides excellent practice CDs and audio files of popular works for choirs and soloists at reasonable cost. CD duplication for use by choir members is permitted.