Start your search for buried digitized treasure (print, video, audio, etc.) on the internet at the Digital Public Library of America, a truly noble effort to "Make millions of materials from libraries, archives, museums, and other cultural institutions across the country available to all in a one-stop discovery experience."
In reading the fine print and the footnotes and references in your secondary sources, you may get names of organizations and resources that are housing primary sources. These kinds of resources are constantly being digitized and made available over the internet. So, you can use Google to see what is available from any particular group.
Another great way to identify digitized primary sources with a particular geographic focus is to the visit the website of the main public library in that area. Local university libraries will also often have research guides that will identify what can end up being real treasure troves of digitized information.
TIPS for efficiently searching Google:
You can find primary sources through a Google search attaching words or phrases ("primary sources" or documents to a search:
navajo code talkers "primary sources"
navajo code talkers documents
The various museums and collections that comprise the Smithsonian Institution, the National Archives, and the Library of Congress have created some marvelous internet tools, exhibitions, collections and databases. A quick way to "get in" is to construct complex Boolean search statements to use in a Google search following these three patterns:
word (or phrase enclosed in " ") site:si.edu
word (or phrase enclosed in " ") site:loc.gov
word (or phrase enclosed in " ") site:archives.gov
For example, if I wanted to see what was available about the Navajo code talkers during World War II, remembering that "less is often better searching", I would do these three separate searches in Google:
"code talkers" site:si.edu
"code talkers" site:loc.gov
"code talkers" site:archives.gov
and I would find some incredible primary resources!
SPECIAL NOTE: If your research has a California focus, you have a truly unique resource available to you in the Online Archive of California which includes primary source collections from over 200 institutions, libraries, archives, historical societies, museums, including the Bancroft Library at Berkeley. It's growing all the time, too!