ChoralWiki, your one stop shop for all public domain music. :)Where can I find SATB music sheets for chorals?
If you're looking for free music, try the first couple of links. There's resources online for you to find royalty-free music.
If you want to order music, try the last link. It's a list of publishers you can order octavos (scores) from.
If you're really feeling adventurous and want music, contact a local university or college with a music department and ask for composers. You might find a student looking for publicity (like myself), or even a professor. Some day, you might be able to say you premiered a famous musician's work!