Most music scores in the library are kept at the shelfmark 780.265
However, jazz scores are kept at 781.652655 which keeps them near the jazz books.
There are a number of websites that provide online access to scores and sheet music in one form or another. Here are a selection.
The sheet music in this digital collection has been selected from the Sheet Music Collection at the John Hay Library at Brown University. The full collection consists of approximately 500,000 items, of which perhaps 250,000 are currently available for use. The sheet music, primarily vocal music of American imprint, dates from the 18th century to the present day, with the largest concentration of titles in the period 1840-1950.
A freely accessible online database, which holds tens of thousands of pieces of classical sheet music scanned from out of copyright sources. Scores can be downloaded as PDF, but note that they can be slow to load due to size.
The Sheet Music Consortium provides tools and services that promote access to and use of online sheet music collections by scholars, students, and the general public.
UCLA Music Library - Contemporary Music Score Collection
The collection includes the digital, open access scores from the Contemporary Score Edition series, the first open access edition of new music published by a library, and scores from the Kaleidoscope 2020 Call for Scores, an open access collaboration with the UCLA Music Library.