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 - Just search by the composer's name in your browser.
Many libraries have been scanning scores for years. Here are a few examples.