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 botanical and natural history drawings and prints. One way to find early print or folio 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.
The Ricker Library for Architecture and Art has a wealth of folios, facsimiles, and other books that contain plant designs, illustrations, patterns, and ornamentation for art, building details, interior architecture, graphic, and textile design.