You might optionally look for primary sources about historical events, periods, people, or social conditions depicted in the text you are researching. Think of words that represent those concepts. Using "Dear John Wayne" as an example again, imagine a researcher interested historical battles between Native Americans and white people like the one depicted in the movie within the poem. The researcher might choose terms like:
Indians, natives, Sioux
Settlers, pioneers, soldiers, cavalry
At the heart of thinking of search terms is the idea of anticipating what words might appear in an article about your topic. Notably absent therefore are terms "Native Americans" or "First Nations." Those terms weren't used as frequently in the past as they are today. Primary source research challenges you to think of terms that would have been in use during the time period you're researching. This can even mean using search terms that today are offensive, but that in the past were common.
These are collections of primary source materials. They are useful for finding sources that were published during the historical period you are researching.
These resources are digitized archival collections; that is, historical materials that have been photographed or scanned so that they can be searched for and viewed online.