Finding Scores Online
International Music Score Library Project (IMSLP). A FREE collaborative music score library. Over 15,000 works.
Links to Digital Scores in the Online Resources for Music Scholars (Harvard University)
RISM (International Inventory of Music Sources) Here you find RISM’s vast collection of musical sources and can freely search among 700,000 entries that contain mostly historical music manuscripts (the majority originated before 1800).The original sources are available from the libraries, music archives, and private collections as indicated in the RISM database. These institutions can often be approached for reproductions.
Most musical scores are housed in 2nd floor of the Leah Peterson Learning Commons.
Call numbers often begin with the letter "M."
Including the word "score" along with the composer may help you narrow your search to find the result you are looking for. Here's an example of a search result using the keywords "Rossini" and "score"