SFS Kids:  Fun with Music - San Francisco Symphony
http://www.sfskids.org

Smithsonian Jazz Class
http://www.smithsonianjazz.org

Rock and Roll Hall of Fame
http://www.rockhall.com

Institutional Collection: Musical Scores – Sibley Library, Eastman School of Music, University of Rochester
Thousands of music scores and books in the public domain.
https://urresearch.rochester.edu/viewInstitutionalCollection.action?collectionId=63