How can I find out the number of books sold by a certain author? Specicially, lesser known writers.?

I am researching writers that you could refer to as conspiracy theorists i.e, David Icke or Milton William Cooper. I am looking for statistcs on the number of books they have sold to get an idea of readership. I know, for instance, that these books can by purchased on the shelf at Borders, so I am uming there is a supply and demand here. Any thoughs on how I can determine these numbers, for any author really.