Not really difficult to find, Training Shooting Sports by Barth/Dreilich is in print and available from the NSRA or on Amazon or a well known auction site. Pistol Shooting by Standl, and Successful Pistol Shooting by Frank & Paul Leatherdale are available at a variety of prices.
Apart from the Barth and Dreilich book, the other two are really not very useful. The rules have changed considerably since they were written and training techniques have advanced considerably. They are OK from a historical perspective, but if you are interested in performance improvement then you need Pistol Shooting The Olympic Disciplines.
You may wish to factor this into your pricing.
Rutty