- Public Domain Resources
As music loses its copyright, it is posted through online open-source sites. These sites can be valuable resources to teachers who need to find a classical text out of print, or simply working on a tight budget.
The Lied, Art Song, and Choral Texts Page - Public Domain Choral Scores
http://www.recmusic.org/lieder/
The IMSLP / Petrucci Music Library - "The free public domain music library."