Just google for some shifters. Any stand-alone shifter should be compatible with any wheel. (IE the ActLabs shifter will be, the Logitech G25 shifter won't work unless you have the full Logitech G25 wheel, pedals and shifter). If you're the DIY type, you can build your own if you take apart an old gamepad or something!

Well, you can buy the "ACT Labs GPL Shifter" which is quite good, but your best bet is just to get a G25, as a shifter with no clutch is no fun!