Lester S. Levy Collection of Sheet Music
Part of Special Collections at one of the libraries at John Hopkins University. The Lester S. Levy Collection contains nearly 30,000 pieces of sheet music; the focus is popular American music from 1780-1960. An image of the cover and each page of music will be retrieved if the music was published before 1923 (in the public domain).
Link to resource:
Lester S. Levy Collection of Sheet Music
Fulltext available: Primary Sources (Historical)