To access the library's physical collection of printed music scores, use the Library's Catalog.
In library catalogs, pieces of music are given added, preferred titles, so that one search finds every version of a piece. Preferred titles (aka uniform titles) are especially useful because pieces could have:
This tutorial from the Indiana University Music Library is a good introduction to preferred titles for music.
RISM Series A/II: Music Manuscripts after 1600, published by the Répertoire International des Sources Musicale (International Inventory of Musical Sources), is an annotated index of music manuscripts held in libraries, archives, monasteries, schools and private collections around the world. Other publications of the RISM project are in book form and may be found in the Music Reference section on the 7th floor of Bobst Library, call number ML113.I67.