Use bibliographies. At the end of most professional journal articles, you will see their list of references. Use those to find more sources relevant to your topic
Follow up on authors. Finding other articles by the same author will likely lead you to other valuable resources. Some ways to do this include:
Note terminology used. When you find one good hit, look at subject headings, descriptors, key terms, etc. for other search ideas.
Keep a search log. Nothing formal – just jot down the searches that you’ve tried (and where), along with author names, additional search terms, etc. It will help keep you from repeating yourself and will provide you with a growing list of search ideas.
Think outside the box. Don’t limit yourself to articles on your exact topic. Try to look at the bigger picture issues that contribute to your question.