On iStock, you can see it in your stats.
For me, it's c25% - 30% US, 25% - 30% UK.

On Alamy, at least for RM files, you can see it in the sales details for each licence sold.

I think I've read you can see it in your SS pages too.

My understanding of the original question was for buying overall rather than for individual contributor experience. If so, that is pretty much ramked from the USA down by GDP, but Shutterstock did produce some interesting stats for 2017 (maybe they have done it since also) at https://www.mystockphoto.org/shutterstocks-contributor-earnings-report-2017/

For individual contributor results it depends what you shoot. As a dour, miserable Scot I shy away from happy shiny people. In fact, I shy away from people altogether and take more pictures of buildings. Consequently my under saturated, under exposed, grim pictures sell better in Europe than in North America and, for reasons I cannot explain, seem to do well in South Korea.