I just stumbled across this terrific resource for finding primary sources on the Web. It comes from the American Library Association. It has great suggestions for places to look, and also gives guidelines for what to look for when you are searching around for primary sources. I highly recommend it!
http://www.ala.org/ala/mgrps/divs/rusa/resources/usingprimarysources/index.cfm
http://www.ala.org/ala/mgrps/divs/rusa/resources/usingprimarysources/index.cfm