I actually bought two Logitech Cordless Rumblepads off eBay (average cost was about $13/pad) for the purpose of making a cordless CP. Problem was, the more I started working on the CP, the more I decided I really wanted a full-on CP with spinner and trackball, so I went with a 2-player CP and a USB minipac.
...but I kept the gamepads for p3/p4, and for a few special games (having the two analog sticks and buttons as well is actually kinda nice for some odd games like Assault or Battlezone, and the "flipper" buttons are good for Vindicators) since I don't have top-fire joysticks on my CP.
I would like to find a low-latency wireless USB doohicky, though... but I sure don't know of one.
-->VPutz