Start with the country profiles created by the United Nations Statistics Division (UNSD) and browse the brief history to determine when censuses were taken.
Each profile contains:
If you don't see the name of the country that you are looking for in that list go to the historical bibliographies tab in this guide. The UNSD site only contains current countries.
Use the general statistics tab in this guide. Note: these resources provide a mixture of census, administrative, and survey data.