• Historians carefully document their sources in footnotes and bibliographies. You can get a good sense of not only specific primary sources that may allow you to explore a topic, but you can also get a general sense of the kinds of sources historians have used to research a topic.
   
• From the secondary literature, you can identify individuals, places, institutions, and organizations important to your topic. This information will assist you in searching for primary sources in online catalogs.

There's no one thing, but for historians, it's generally something that was created at the time of the event. Try searching using terms like: biography [also use for autobiographies], newspapers, "personal papers" [use for archival material], correspondence [use for letters], personal diaries, photographs, videos, interviews, pamphlets, artwork, sources [a common one!], and many other types of things!