Dissertations: If you find that someone has written a Ph.D. dissertation on your topic, you can be absolutely sure that the author did exhaustive research on the topic. The dissertation will include a lengthy bibliography to document that research effort. Use this existing list of resources as a starting point for your own research.
To find dissertations, use the database Dissertations & Theses Online. Almost all of the dissertations published in the last decade are full text online.