MPJA is selling Atari Trak Ball controllers for US $1.95 each. The minimum order they'll process is US $15, so you'll have to purchase eight of them! I had a hard time finding this item on their web site, so quote catalogue item number 6465 when placing your order.
Purchase a new Jaguar system for US $33, from Dentec, located in Scarborough, Ontario, Canada. They also carry controllers, and a decent selection of reasonably priced cartridges, including hard to find titles, like Ruiner Pinball, Pitfall: The Mayan Adventure, and Fight for Life.
Video Game Liquidators has some good deals on Jaguar systems, controllers, accessories, cartridges, and CD's. They're also offering a free copy of the Official Jaguar Gaming Guide, with every order. Their Jaguar page, lists everything they offer.