According to the United States Code, Title 44, Chapter 19, Sec 1901:
A government document is "informational matter which is published as an individual document at government expense, or as required by law"
A government document can take on my different forms such as:
Microprint, CD-ROMS, Books, floppy discs, journals, websites
Best place to find to government information is a Federal Depository Library which receives publications free of charge and makes them available to the public.
Other places to locate government information are the nation's National Libraries:
Library of Congress
National Agriculture Library
National Library of Education
National Library of Medicine
Presidential Libraries and Museums