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)