Consider what types of primary sources might have been created during the historical moment you are exploring and that might be relevant to any subfield you are exploring.
One of the best places to start looking for primary sources are secondary sources that you've located. What are the author(s) citing as their primary source evidence? Do you have access to the same material?
Make sure you leave yourself plenty of time for primary source research as it can be the most time consuming part. If you need help, reach out to the library!