Including these keywords in your search can help you find primary source material:
Also, look for these words in subject headings, and follow the subject links.
The U.S. government is one of the most prolific publishers in the world. Nearly every aspect of life is touched by government activity, and it is a publisher of all types of materials (statistics; reports; legislation; etc).
Visit their home pages. Have they digitized materials? If they haven't digitized, have they published Finding Aids (which give you a good sense of what materials are contained in their collections)?