The easiest way to find relevant primary sources is to find them within your secondary sources. They'll be in the citations, as scholarly sources in history use and cite primary sources, and they'll be part of the historical narrative. Look for references to documentary and other primary evidence in your events (a speech was given, a law was passed, a treaty was signed, etc) and look for clues that can lead you to primary sources: newsworthy events, key actors who may have authored an autobiography or reflections, representative images and more.
Once you do your preliminary research and understand the background of your topic, including the key people, places, events, and dates involved, you can start to think about finding primary sources in addition to those found within your secondary sources. Ask yourself:
Here are some frequently used categories of primary sources: