Where I can get information on how many copies a particular book may have sold?

I was thinking that this could be useful information in trying to estimate the size of a particular target market you'd like to reach....

For example, if you'd like to reach people who love Great Danes (a breed of dog), you might be able to estimate the potential size of this market by the sales of mainstream books about Great Danes.

Of course, this kind of info might be difficult to find (short of mailing lists from the SRDS or other direct mail sources similar to that) - I don't know....

Thanks for any feedback.

- Dien