Search the library's catalog.
Click Advanced Search to the right of the search box.
Here you will select the Library Catalog radio button.
Type in your search term.
Example: Renaissance Music
There are a lot of research results. Look at the column on the left and you will see Resource Type.
Select Scores and then click Apply Filters at the bottom.
You have narrowed your research results to Renaissance Music Scores!
Some websites that have free music scores
IMSLP (International Music Score Library Project
Library of Congress Notated Music (browse by year)
All the collections of music at the Library of Congress
Individual Composers Websites
Just search by the composer's name
Many libraries have been scanning scores for years. Here are a few examples.
Indiana University Cook Music Library
Sibley Music Library, Eastman School of Music
Borrowing Scores from other libraries
WorldCat is the OCLC union catalog, containing more than 43 million records describing books and other items owned by libraries around the world. Each record contains library holdings. WorldCat covers thousands of subjects and includes records for items as far back as 1000 B.C. Materials covered in the catalog include: books, internet resources, computer data files, computer programs, films, journals, magazines, manuscripts, maps, musical scores, newspapers, slides, sound recordings, and videotapes. WorldCat does not include individual article titles or stories in journals, magazines, newspapers, or book chapters.