If you can't find it in RISM, try these other resources:
Digital Resources for Musicology (Stanford University) lists information by category, with Digitized Music Manuscripts at the top. The focus is on open-access projects.
Digital Scores: A Guide to Online Notated Music Sources (Princeton University) aims to collect resources about finding notated music (scores and parts) online, whether commercial or free.
Europeana, a European-wide digital library
Online Resources for Music Scholars (Harvard University): Links for research on a wide variety of topics in music, including historical musicology, ethnomusicology, music theory, composition, and performance practice.
ViFaMusik (Bayerische Staatsbibliothek): a single search engine for music and musicology
WorldCat.org, a catalog of holdings from libraries around the world
Clori, Archivio della cantata italiana
DIAMM, Digital Image Archive of Medieval Music
Gallica.fr, the digital library of the Bibliothèque nationale de France
Paul Sacher Stiftung for music from the 20th and 21st centuries