Human population growth - where can I find the data?
National censuses are collected, published and disseminated primarily by each country's census or national statistical office (NSO) or its equivalent. Census data are published in each country's vernacular, and as appropriate, many offer English as well as other language versions.
Statistical offices of international agencies (e.g. United Nations, statistical organizations for member states like the ASEAN, Eurostat, and OECD), also publish statistical data including population figures. The most consistent data in terms of format and frequency, whether online or in print are usually generated from them.

The US Library of congress provides some interesting links here:
http://www.loc.gov/rr/business/census/intlcensus.html
Also Cornell provides a few resources also:
https://www.ciser.cornell.edu/ASPs/datasource.asp
If you still need some information, your best bet is to then contact social scientists and ask for their opinion.