To find manuscripts and early editions of scores or music literature (books, treatises, etc.), you will need to find out what exists and then figure out whether other versions are available besides the original(s). Though many materials exist only as unique items in libraries or archives, the proliferation of online versions and reprints has made it possible to conduct much research without consulting the actual original documents.
To find out what exists, use these tools:
The composer (Consult the Works list within the article.)
"Sources of Instrumental Music to 1630"
"Sources of Keyboard Music to 1660"
"Sources of Lute Music"
"Printing and Publishing of Music"
To find the source itself, use tools in the Facsimiles & Digital Copies tab in this guide.