If you can't find it in RISM, try these other resources:
Digital Resources for Musicology (Packard Humanities Institute, 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
Music Treasures Consortium (Library of Congress): A consortium of institutions worldwide that presents digitized primary source manuscripts, early printed editions of notated music, early treatises on music, and correspondence of prominent composers.
musiconn – Für vernetzte Musikwissenschaft (Bayerische Staatsbibliothek): a single search engine for music and musicology
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.
vdm16: Verzeichnis deutscher Musikfrühdrucke/Catalogue of early German printed music focuses on printed items containing notated music published in the German-speaking lands from the 1470s to the mid-16th century.
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
IFMuC (Inventari dels Fons Musicals de Catalunya): Catalan Musical Sources Catalogue (Universitat Autònoma de Barcelona)
Paul Sacher Stiftung for music from the 20th and 21st centuries