Where can I find sources for historical data on Buddhist populations in various regions around the world?

I need some sources regarding the changes in the size of Buddhist populations since the death of Gautama Buddha, e.g. the number of Buddhists in what is modern day India from 410 BC until now, or just global historical data of Buddhist populations around the world since the religion started spreading. Exact sources would be great. This is part of a research paper I am doing, and partly out of pure curiousity as well. Thanks.