Historic American Sheet Music - Duke University's Historic American Sheet Music Project provides access to digital images of 3042 pieces of sheet music published in America between 1850 and 1920 - www.scriptorium.lib.duke.edu/sheetmusic

The Lester S. Levy Sheet Music Collection - A large collection of sheet music housed at Johns Hopkins University. Online searchable database, including images of works in the public domain levysheetmusic.mse.jhu.edu/index.html 

Music for the Nation: American Sheet Music, 1870-1885 - Tens of thousands of songs and instrumental pieces registered for USA copyright in the post-Civil War era, from the Library of Congress. Searchable with images - www.memory.loc.gov/ammem/smhtml/smhome.html

19th Century American Sheet Music - A collection at the UNC-Chapel Hill Music Library, still in the process of being scanned and indexed. Some images available - www.lib.unc.edu/music/eam.html