http://www.amazon.com/NYKO-TECHNOLOG.../dp/B0006NGY9C
$11.99 is a very good price for something having this device's reputation for reliability. Reviews for it are strongly positive. I had the TechnoKats purchase a couple of them for development work, and they do seem well-built with an impressively long and sturdy USB cable. The cooling fan might be a gimmick, but a couple of the students thought it was a very good idea.
The range of motion on a game controllers thumb-activated "joystick" is extremely small. I suspect we'll have to come up with some fancy software to make it easy for the driver to control the robot well using them.