Choral Public Domain Library (CPDL) has over 12,000 online score published before 1923.
Internet Music Score Library / Petrucci (IMSLP) includes 380,000 public domain scores in all genres from before 1923, as well as scans of primary sources uploaded by the world's great libraries. Browse the Genres/Instrumentation guide for choral music or go to the Home Page to search for a specific piece. (Non-members encounter a delay when downloading scores)
The Web Library of Seventeenth-Century Music is a scholarly series published by the Society for Seventeenth Century Music. Many of the works are for choir. This open-source series is scholarly and also practical. It includes parts for orchestra and keyboard when indicated.
Boosey & Hawkes offers free online perusal of scores by important modern composers including Adams, Andriessen, Floyd, Reich, Rorem, Stravinsky, and others.
Benson Collection of Hymnals and Hymnology at Princeton Theological Seminary includes over 6,000 books and hymnals, most from the 1800s.
OneSearch includes scores available through HathiTrust. After starting a search, limit to "Full-Text Online" and then select "Book / ebook." HathiTrust does not distinguish among content types.
Use OneSearch to find printed music (scores) in the Music Collection:
Printed music and books are located in the "Music Collection," which is part of Education, Music and Media on the Lower Level - West. Use OneSearch, on the Libraries' main page (http://www.bsu.edu/library). Start with keywords, including a composer, title, or number:
From the preliminary results, click "Library Catalog." You will then see the formats ("content types") available: