The EyeLink Portable Duo operates with USB XBOX controllers. SR Research has evaluated the timing properties of a number of controllers and the best we have found is the Logitech F310 Gamepad in DMode which has a delay of about 5 msec and variability of only a few milliseconds. See the response distribution below.
This device is included with the EyeLink Portable Duo.
For even more accurate and precise timing, SR Research Ltd recommends the BlackBoxToolKit USB Response Pad on the Stimulus Presentation Computer/Display PC. These devices are millisecond accurate and are recognized as keyboards, so can be used with any operating system or programming approach.
SR Research can supply two different 5 button layout (W and V as pictured above) of the BlackBoxToolKit Response Pads. Both have large arcade-style buttons with clear key caps that can be removed to place your own information beneath. The device has TTL out as well as USB.