A keyboard is much better for shooters in my opinion, but if you must... look for the Logitech Dual Action USB, you can get it both with and without the rumble effect.
Ayy....I dont like game pads and I dont know if you can use them on that game, but the one he suggested is...OK, i dont like them that much. You can have mine for $5......with software.......
ive used a zboard keyboard for awhile now, fps keyboard tileset works pretty nice I guess. if you really want a gamepad, I think zboard makes a little pad close to the one someone linked above.