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

By topic:

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)

Internet Culturale and ICCU / SBN catalogo from Italy

Paul Sacher Stiftung for music from the 20th and 21st centuries