Write down synonyms or keywords for your topic. Use these in your search. Can't think of synonyms? Look at the words used in the abstracts of articles or books that you find.
Try to find "core" sources or sources that are considered landmark or seminal.. Not sure if you have a landmark source? Google Scholar can give you a rough idea of the importance of your source: If a high number of articles cite yours, (most of the time, but not always) it is considered important. Paste the title of your article in Google Scholar search box. Look at the number of other articles that cite your article title: