Some sources that may interest you: The Federation of American Scientists (fas.org), the Strategic Studies Institute (strategicstudiesinstitute.army.mil), OpenCourseWare (ocw.mit.edu/courses/political-science/), the Center for Strategic and International Studies (csis.org).
Reddit /r/geopolitics maintains a wiki with a list of open source references that you may find interesting: https://www.reddit.com/r/geopolitics/wiki/index
There's too many things to recommend: foreign news outlets, official press briefings, leaked documents, academic and military history, think tanks and their analysis.
Basic advice: read everything and don't believe any of it.