For historian's interpretations, JSTOR is amazing, and you can often access it through your school. If you find an article about the topic you want, putting it into JSTOR's Text Analyser tool can help you find more articles on similar themes. For primary sources, its a bit more difficult. If any contemporary books, pamphlets or other sources are frequently mentioned in your reading, those would be useful to look at. For example, Luther's 95 Theses might be a good source.