"Grey literature" is defined as "Information produced on all levels of government, academics, business and industry in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body." (Luxembourg, 1997 - Expanded in New York, 2004). It is often more difficult to find and obtain than the published literature.
It may include such things as conference procedings, policy studies and reports, white papers, annual reports, draft legislation, think-tank reports.
Below are some resources to get you started. Also think about relevant websites from organizations and keep in mind conference proceedings that may be published in association journals or accessible from professional association conference websites.