This is a collection of internet sites I use to find vintage sheet music. I am no way affiliated with these sites, but use them as a valuable music resource. If you know of an on-line sheet music library that is not listed here please let me know and it may be included here.
Brown University Center For Digital Scholarship
cdbpdx : This is a large collection of 78 rpm recordings (No sheet music)
Charles Templeton Digital Sheet Collection At Mississippi State University
Cylinder Preservation and Digitization Project at the University of California, Santa Barbara
(a collection of cylinder recordings)
Duke University Digital Collection
Halcyon Days Music : complete lyrics, but no musical notations
IN Harmony : Sheet Music From Indiana
The Maine Music Box Collection
Ohioana Library : preserving the work of Ohio artists
John Hopkins University J Scholarship site
National Library Of Australia : Fabulous collection from down under
New York Sheet Music Society
Songbook1 : At wordpress, there is unique historical and biographical information too.
University of South Carolina Sheet Music Collection