American Country Data via CountryWatch
Historical Statistics of the United States: The standard source for the quantitative facts of American history.
Historical Statistics of Canada: Contains about 1,088 statistical tables on the social, economic and institutional conditions of Canada from the start of Confederation in 1867 to the mid-1970s.
Search the Internet to find the government agency in each country that produces population statistics. The United Nations provides links to those agencies here: http://data.un.org/
And visit these sites that compile statistics for countries of the world.
CountryWatch: Provides demographic, political, economic, business, cultural and environmental information for counties of the world.
International Database of the U.S. Census Bureau
UNdata: A central repository for statistics the United Nations collects.