George Mason University has launched the Mason OER Metafinder, designed by GMU Librarian Wally Grotophorst, Associate University Librarian for Digital Programs and Systems. It searches a broad range of Open Educational Resource Collections, such as as the Digital Public Library, the Library of Congress American Memory Collection, MERLOT, and College Open Textbooks. Read more about the Metafinder at Deep Web Tech Blog, or try it out below.
Do a search for your topic using the Everything search box. Use "quotation marks" around phrases.
Then on the left of the search results under Format, check Downloadable Article and Downloadable Archival Material. You'll get open access and institutional repository content from all over the world.