I’m writing a review on digital collections of sheet music. One frustrating aspect has been simply locating these collections. The Sheet Music Consortium lists the collections from which they harvest metatdata (i.e., collections that you can search using their tools), but they do not distinguish between collections that include page images and those that do. Not every institution with a digitized collection participates in the Sheet Music Consortium so the list is incomplete. Duke University has an extensive list, but they don’t always indicate whether or not there are images. And the list has quite a number of broken links.
I have compiled a list of digital collections of sheet music here. I hope that others will contribute to and correct this list so that it is a good resource for researchers, educators, students, and others interested in the sheet music from the 18th, 19th, and 20th centuries. Just leave me a comment here if you any additions or corrections.