Many universities, libraries, and museums have digital collections that include primary source material. Linked below are a few suggested starting points, but there are many other useful sites out there.
Search the Library of Congress's online American History collection
Search tips for finding sources
Try adding "primary sources" to your topic keywords in Google or Bing to find source material.
You can limit your search to government websites by adding "site:.gov" to your search.
You can limit your search to College and University websites by adding "site:.edu" to your search. Remember, many schools have web space for students to post projects and presentations. Only use an educational site if you are certain the page is not a personal page.