Publishers keep sales figures very close to the vest. Unless you have access to Neilsen Bookscan, you really aren't going to be able to cite sales figures for other books.
There are other indicators of popularity. Was the book on any bestseller lists? Did it get special press coverage? Has the author written other popular books?
A bigger question is why you need this information. Identifying competing works is often part of nonfiction proposals, but if you're submitting a novel you shouldn't have to do this, at least for a commercial publisher. A simple query letter is sufficient.
- Victoria