Music like that, depending on how old, is making its way into university collections. It is scanned in and in some instances the first few bars are run through a recognition program to offer a tinny file letting you know what the tune sounds like. Finding the university or special collections that wants it, that's the difficult part. If the prints are too new, even of older works, then the copyright issues kick in (as you no doubt know).

Collectors of ephemera love the old sheet music, especially with more elaborate covers and even better if it was printed in four-colors (offset). Look at eBay, and you might try searching some of the titles through Bookfinder or ABE books - they deal with printed matter beyond just books.