Scores are organized by the instrumentation and ensemble size. For example:
M3-3.3 Collected works of individual composers
M5-1480 Instrumental music
M6-175.5 One solo instrument
M177-990 Two or more solo instruments
M1000-1075 Orchestra
M1470-1480 Aleatory music. Electronic music. Mixed media
M1497-1998 Secular vocal music
M1500-1527.8 Dramatic music
M1528-1529.5 Two or more solo voices
M1530-1609 Choruses
M1611-1624.8 One solo voice
M1999-2199 Sacred vocal music
M2000-2007 Dramatic music
M2010-2017.6 Choral services, etc.
M2018-2019.5 Two or more solo voices
M2020-2101.5 Choruses
M2102-2114.8 One solo voice
M2115-2146 Hymnals. Hymn collections
At UWM Libraries, and most American university libraries, most physical materials are arranged according to the Library of Congress classification system. In the system, each item is given a call number, which includes a combination of letters and numbers based on the subject matter, author, and date of the item.
Music materials are divided into call numbers beginning ML, for books and other material about music, MT for sources related to music instruction and study, and M, which is used for printed music. Within ML and MT, the organization proceeds by specific subject area. In the Ms, the organization is based on the instrumentation or ensemble the music is written for, with M2 and M3 used for large collected editions that might mix instrumentations.
While there are many online resources for purchasing scores, the resources below are free to you and accessible online.