Try the HathiTrust Digital Library and the Internet Archive for primary source books (memoirs, travelogues, histories, polemics), government reports, conference or meeting records (such as political conventions), and any other published sources.
Try the Library of Congress and the Digital Public Library of America for letters, documents, maps, and photographs, many of which may be downloaded.
Published correspondence is considered a primary source, so if your topic involves a famous historical figure, try our library catalog or WorldCat to see if there are volumes of letters available (go to the "Books" tab of this Guide).
The Avalon Project compiles documents from world history (4,000 BCE to present) on law and diplomacy. (Yale Law School)