Locating specific works in monuments (or even knowing what's in them) can be a challenge. Some of the monuments have individual catalog records for each volume, in which case the library's catalog may direct you to the right set and volume. If not, the best source for this is usually this book:
G.R. Hill and N.L. Stephens: Collected Editions, Historical Series, & Sets & Monuments of Music: a Bibliography (Berkeley, 1997) [ML113.H55 R4880 in the RBI Reference section]
The Hill book lists monuments and other anthologies of music and includes the contents of each set. You can look up the set and see the contents (up to 1997). You can also look up works by composer and find out which sets (and which volumes) include a composer's works. For example, the page shown below will tell you that Hassler's Cantionaes Sacrae is in series 1, volume 2 of Denkmäler Deutscher Tonkunst, and the Canzonette von 1590 are in volume 9 of Denkmäler derTonkunst in Bayern.