I just typed this answer in for another question, but it applies here as well.
I always start by searching for related keywords to see what pops up in Google, Yahoo and Bing. Each gives something different. It seems if I wait a bit between searches, something new will rise to the top, no matter the subject. I search for PDF and video related file extensions in advanced Google search as well. Have foiund a lot of great freebies that way for studying.