The RISM website contains most comprehensive and current list of all RISM volumes and online vs. print availability. The information below is current as of March 2017:
A/I: Printed music issued individually, 1600-1800 [online via RISM website]
A/II: Music manuscripts after 1600 [online via RISM website]
B/I: Printed collections of the 16th-17th centuries [partly online via RISM website – only 1500-1550]
B/II: Printed collections of the 18th century
B/III: The theory of music from the Carolingian era up to c.1500
B/IV: Manuscripts of polyphonic music, 11th-16th centuries [online in DIAMM (Digital Image Archive of Medieval Music)]
B/V-B/XVI: Additional bibliographies of material organized by topic
C: Directory of Music Research Libraries
How to search RISM online:
How to search RISM in print