You can do an advanced library search and limit to "Score" in the Material Type box. By entering a keyword (band, symphony, piano) you will get a large number of results. Adding specific keywords will narrow your results significantly.
If you have a specific composer in mind, do an author search.
Another option is to search by subject in the Field drop down such as Music -- Manuscripts -- Facsimiles OR Music by women composers -- 20th century.
Sheet music can be found on the 5th floor of the Weaver Science - Engineering Library.