Evaluate Journals
If you've found a source that works well for your research, look at their bibliography. There's probably going to be a couple of sources that they've used that you should use as well.
Google Scholar and many of our databases have a "Cited By" or "Times Cited" tool that lets you see what other sources have cited an article. If you've got a good article, see who else cited it. You'll probably find a couple of other sources relevant to your research.
Many of our databases will assign Subjects or Subject Terms to the articles, identifying the main topics of that article. Look at the Subject Terms to see if there are other search terms you should be trying in a database.