The source is a book called "How Much Can I Make" and it is a reproduction of the earnings claims that many franchise companies put in their federal disclosure documents. The book is updated every year to keep it as current as possible.
The bad news is that you'll have to buy the book. The publisher is Source Book Publications. You can call them at 1-800-841-0873 to find out the cost and availability data.
Hope this helps and good luck on your research.