These are selected sources for international data as well as US country level data.
Google Scholar is often helpful for finding articles on your city or country. Some articles will contain statistical content.
These are selected resources for national data.
Most city data (statistics) has to come from offical government websites of the city or country or from statistical abstracts and handbooks of the individual country. Most international organizations and databases generally provide only country level data. Encyclopedias like the Encyclopedia Britannica Online are useful for descriptions of cities and their statistics.