As you find books and articles, be sure to mine their references for sources. This means reading what the author of a source says about other sources in the field. Most scholarly articles have a section (often near the beginning) where they discuss the research and scholarship that inspired their own. Books often have a section like this, too. Pay particular attention when the author says things like "So and so is a key leader in the field" or "So and so's methodology impact our work in significant ways." This is the conversation. Plus, you will probably want to track down some (or all) of the sources that your original source describes as significant.
By tracing cited works, you're drawing on the evidence others have used and may find connections that you would otherwise miss. You will also see the patterns of the conversation emerge: works cited by everyone else are worth a look; authors who write a lot about your topic are worth searching by name, etc. Finally, remember that this is the way most scholars search for sources, so if you also search this way, you'll be searching in a very sophisticated and informed manner.
How to do a bibliographic trace: Search for cited books by title or author in library catalogs; for journal articles, check the Do We Have This Journal by journal name to see if we have an article you want. Several databases also include features telling you how often a work has been cited. Use the Tracking Down Sources tab (above) for more pointers on how to find hard copies. Please also contact me or any other librarian if you need help at any point of this process.
You can (and should) also go forward in time to see who has cited your original source.