Many primary sources (manuscripts, early editions, and reproductions such as reprints, facsimiles, and microfilm) are entered in WorldCat. Once you've identified sources of a work in reference tools like Oxford Music Online, composers' thematic catalogs, and RISM, you can search it in WorldCat and find out which libraries own a copy.
Search the work in WorldCat like you would in a library catalog. Then look for a link "Libraries worldwide that own item."
Reproductions can often be acquired through interlibrary loan.