Unfortunately, my record of sites where I've found sheet music scans is mostly on the individual sheet music that I've saved, so there's no simple way to compile a list of places where I've found things. With the exception of the major repositories that have been mentioned, where I might go to the site to search, it's more often a matter of picking up a Google hit and remembering, when I get to the site, that there may be peculiarities to be observed in getting the bits of information from that site.

Without looking up links (which would be very difficult, requiring that I look at nearly every piece of music I've ever saved) I can recall finding at least three separate collections at Oklahoma Colleges/Universities, at least two in Texas, and some hits into collections in Washington state, Nebraska, and in a couple of back-east places that expect you to "just know" where they are.

I could give you a dump of places I've bookmarked under "Music," but since it's an unmaintained accumulation I'd guess that more than half of the links are no longer good, and quite a few aren't really to anything musical but just happened to get dropped there. (I see several for which I can't figure out why or how they got there.)

John