It depends on the subject area you are doing research for. First you have to know what a primary source is before you know where to search. See this FAQ to learn more.
Many primary source materials are included in some of the Library's subscription research databases such as Academic Search Premier, a multidisciplinary collection. Also, Issues and Controversies in History, is a database that includes overviews of history topics, and related primary sources. These databases allow you to limit your search to primary sources.
The Library of Congress provides access to a wide variety of primary sources arranged in collections by topic.
For medical/clinical research, Trip Database is a free online tool (there is a subscription version) that pulls free content from the web and categorizes it. It will allow you to filter for material categorized, such as "primary research." It can help you quickly scan resources that will work for your assignment, though you could otherwise find the content free online.
