For your lit review, you'll focus exclusively on scholarly sources. Still, it may be worth thinking about the different things scholarly sources do. This is how some folks sort that out - the BEAM method.
Background - you won't cite Wikipedia in your review, obvs, but scholars often provide good background in the intro to their books before they launch into an argument.
Exhibit - also known as primary sources - data sets, documents, eyewitness accounts. Your sources will use these, but you won't include this kind of source in your lit review.
Argument - most of the sources you cite will be making an argument and they will use exhibits (qualitative or quantitative research, primary source documents) to do so. You'll be sorting through and grouping arguments as you identify the gap your research question will fill.
Method - in political science, this is more likely to be called "theory." These sources lay out a way of thinking about things. You might find a lot of your sources drawing on a particular theory. Maybe including one of those foundational works will make sense as you tell the story of scholarship on your topic.
You've done SO much research. You can bang out a paper in your sleep. You know all about plagiarism. You've probably annotated a million bibliographies. Find fifteen sources and write about them? No problem.
A literature review is different. You're not just finding and describing sources on a particular topic, you're doing two things:
Three pieces of advice:
Scholars publish books, book chapters, articles in scholarly journals, and they give papers at conferences that are in some cases published (and sometimes are called "proceedings"). These are some good places to start poking around for research on your question. There is no single place where you'll find everything. Each option has its own frustrations, but they all offer some ways to focus your search results, in particular:
It's easy to sort articles from scholarly journals from a newspapers or magazines, though there are things in scholarly journals (e.g. book reviews) that are not peer-reviewed research and don't belong in your lit review. It's trickier with books. University presses publish popular books to pay the bills, and other presses publish work by scholars that might have a wider audience than just scholars but are not watered down. There's no filter other than your own judgment to separate scholarly books from one that doesn't pass the test of scholarship. Look for books that are
Here are databases to begin your exploration.
You can't do a literature review without paying attention to citations. These are the map of how ideas flow and intersect. It's the LinkedIn of people involved in the conversation about your research question. Look for repetition - who does everyone cite? You probably need to check them out. Look for patterns. People who ask this kind of question use this theorist's work all. the. time. Etc.
The trouble is most citations are broken links. You have to do some work to get those sources. Interpret the form it was published as and then:
See who has cited a work
As you scan references in books and articles, you're looking into the past. You can also see who has cited a work since it was published using Google Scholar (which sometimes also gives you a list of cited works in the Social Sciences Citation Index).
As you dig in, you'll find certain scholars are the experts. Look for a CV (resume) online or a profile of them at their workplace. They may have lists of publications and presentations that are helpful for your research. Maybe they've had a grant from a foundation that supports work related to their agenda. See if they have a public presence on Twitter or Facebook. You'd be surprised how many academics use Twitter to talk about their work with each other, usually with a hearty dose of sarcasm and/or personal reactions to events.