Finding primary sources in archives and special collections can be very different from finding peer-reviewed research in a library database. The links below will help give you a good understanding of how to find primary sources.
Slides from the library session
What is a Special Collection? and What is a Primary Source? from Purdue University Libraries
Distinguish Between Primary and Secondary Sources from UC Santa Cruz Libraries
Special Collections Citation Examples from Wilson Library, UNC University Libraries
Image credit: US National Library of Medicine, original document