Links to sheet music resources
For free downloads of music that is out of copyright:
To find music that is rare, or under copyright that is available for fair use, Worldcat is the international resource for searching libraries. If you enter your location, it will start with a list that is closer to you, and continue as needed. Sometimes one must pay for a scanned copy, sometimes works are available through interlibrary loan. If you copy the information from where something is located and all identifying information from that host institution (call number, file info, etc.), one can take that information to a local library and get their help in requesting the score.
Once on the page, select advanced search (a little graphic to the right of the search window), and in “format” select Musical Score, and enter title of the work you are looking for and the composer’s name (as author). Sometimes it takes a few tries to find what you are looking for (particularly the manner in which you enter the title).