LIBRARIES, MUSEUMS & UNIVERSITY WEB SITES
Digitizing special collections is a huge trend with colleges, universities and museums. If you or your professor know of a place that specializes in a subject or time period you are researching, try looking at their institution's or library's web site to see if they have digitized collections or what they might call a "digital or institutional repository."
Local Historical Societies also tend to digitize collections or finding aids to their collection.
The Library of Congress has a wealth of digitized collections containing everything from print to audiovisual items. Though heavily focused on American History, if you "Browse by Topic" you will find several links for collections with an International or World History focus.