There are many freely accessible open access digital archives where you can find primary sources. This is by no means an exhaustive list. Check with a librarian to find more that relate to your topic. See the Primary Sources research guide for more information.
The Library of Congress American Memory- "American Memory provides free and open access through the Internet to written and spoken words, sound recordings, still and moving images, prints, maps, and sheet music that document the American experience."
The Avalon Project: Documents in Law, History, and Diplomacy- A digital document library by the Yale Law School Lillian Goldman Law Library.
AD* Access- Duke University Libraries, "Over 7,000 U.S. and Canadian advertisements covering five product categories - Beauty and Hygiene, Radio, Television, Transportation, and World War II propaganda - dated between 1911 and 1955." and Emergence of Advertising in America: 1850-1920-"Over 3,300 advertising items and publications dating from 1850 to 1920, illustrating the rise of consumer culture and the birth of a professionalized advertising industry in the United States."
Perseus Digital Library- Digital library of ancient Greek and Roman texts and images from Tufts University.
Duke Papyrus Archive- "The Duke Papyrus Archive provides electronic access to texts about and images of nearly 1400 papyri from ancient Egypt."
Backing up a claim with statistical data and empirical evidence lends credibility to your argument.You may cite other scholars substantiated claims to support your own but it still important that you demonstrate the validity of your own claim and the claims of others with evidence.
U.S. Government Sources:
USA.gov (use this for a general search if you are unsure of which agency covers your topic)