Use the language the scholars use: Look for words and phrases that are closely related to the ideas you are looking for, but which may be different from those that you tried when searching. These can open up a world of additional, useful materials.
Use your bibliography: If you find one really great source, look at the sources that it draws from. This is a great way to follow the scholarly thread of conversation back to other authors, and other writers.
If you really don't know where to begin, try some of the following databases, which cover ALL subjects. This might give you an idea of where to start.