Some Sources for Historical Sheet Music
In the business of putting on historical events with appropriate music, one frequently encounters the need to find appropriate sheet music. Further, when one has been doing historical events for many years, one can get a bit tired of the same tunes all the time, and want to dig up something new--which is to say something that everyone hasn't heard yet.
Fortunately, there seems to be a growing collection of sites on the web that can be tapped for historical sheet music. Of course, with copyright laws being what they are, you aren't going to find much past 1925, but if your interests lie in the Victorian or Ragtime eras (and occasionally earlier) there's some good stuff out there free of charge.
Here are the ones I know about. If you know some more, please add them via "comments".
Sheet Music Consortium
A Multi-library project, hosted by UCLA
Petrucci Music Library: A wiki devoted to public domain music.
Mostly classical but some early 20th Century popular stuff
19th Century California Sheet Music from UC Berkeley
The Baylor Collection of Popular American Sheet Music
19th & early 20th Century American music
Brown University Sheet Music Collection.
Includes "African American Music, 1820-1920", "World War One Music" and "Yiddish American Music"
University of Colorado Sheet Music Collection
The Lester Levy Collection at Johns Hopkins.
1790 - 1980, though the focus is 19th and early 20th Century America.
Library & Archives: Canada
Sheet music from Canada's past. Mostly popular music in English and French.
University of Pennsylvania (1790-1895)
University of North Carolina.
19th Century American Music