The Rare Book and Manuscript Library (RBML) is the University of Illinois Library system's main repository for early manuscripts, rare books, and literary manuscripts in the broad fields of literature, history, art, theology, technology, theater, and the natural sciences. Their collection includes medieval and early modern manuscripts. One way to find early print or manuscript materials is through the search page on their website.
You can also search for manuscripts through the University of Illinois Library Catalog. Once you enter a search term in the box, the search results page allows you to apply a variety of filters to your results.
Select "Rare Book and Manuscript Library" as the location to find books in the RBML collection:
You can also filter by Era to find items from a certain time period, or by Genre to find items of a certain type - incunabula, Books of Hours, illuminated manuscripts, etc.
The Newberry Library is a world-renowned independent research library in Chicago which offers readers an extensive noncirculating collection of rare books, maps, music, manuscripts, and other printed material spanning six centuries.
The Newberry has several online research guides to their medieval manuscript holdings.