Once you have found a good starting source, it's easy to find more! Many search tools provide options to find related/similar items.
Academic Search (EBSCO):
Backwards Reference Searching
What it yields:
It looks into the sources that your author referenced. This means the sources have already been vetted by your author for credibility, and you know that they are related to the topic at hand. However, it also means that the sources must have been published before your original source, so you will be retrieving older materials.
How to do it:
Tools to use:
Most scholars tend to specialize in a certain area. It's likely that they will have authored other items about similar topics. There might even be multiple publications about the same study or findings.
So instead of searching by keyword, search with your author's name to see what else he/she has done!