A few sources that might be useful if you've run out of ideas, or if you just need a break
Google Scholar, Records and citations from Google's Web index calculated to be scholarly. Although useful for most topics, it still contains a small subset of available publications; other databases on this page will have additional items on your topic.
Note: You never have to pay to retrieve results. The full text of some results are linked, but for others the Citation Linker can be used to track them down.