(these tutorials feature pre-1980 primary source finding examples, consult your own research assignments for your specific primary date requirements)
Other Key Resources: Google Books, Hathitrust
You may also find it possible to locate historical primary sources in the bibliographies of the articles and books you've previously collected. These are generally found at the end of the book/article, or included as footnotes. Finally, the Libraries' offer another more general primary source guide with a host of resources beyond what is listed in this guide.