Kelvin Smith Library
Here's a very helpful online guide from University of Illinois Library website:
This guide defines what primary sources are, and describes effective search techniques for locating them in catalogs, indexes, and online archives.
Note that many of the reference resources, citation index collections and online primary resource archives listed on this guide are also available through the KSL website.
Also be aware that there are other physical and electronic collections in the Case Libraries system that provide lots of primary sources. Locations for these resources are indicated in Case Libraries online catalog records.