What exam board are you using? A lot of people have multiple titles, so they may be a political scientist and a historian. Additionally, I find it strange that the teacher said your sources have to be from historians - OCR at least puts a lot of emphasis on primary sources as well as secondary, which obviously won't be from historians, and there is a lot to be gained from the analysis of political scientists and other secondary sources. I wonder if your teacher put that restriction in place to prevent people getting random sources from reddit and so on. In other words, she meant that you should use professional sources that have been peer reviewed and are by experts in the subject, not necessarily specifically historians. Maybe it differs by exam board. Or maybe my school was wrong and I have completely failed my coursework lol.