To find official government population and other statistics, visit the US Census Bureau's page: International Statistical Agencies.
In addition to these sources, each country has its own statistical agency (here is a list of links) and many government ministries also provide statistics on their websites.
The websites of international agencies and organizations such as the UN (including UNESCO), the World Bank, the IMF, and the WHO contain important profile information and statistics and are often a good place to gather information on population, health, education, economic, agricultural and other statistics.
General profile resources are listed below.