Summit, Jegs or Mcleod Direct. You need a 1373 Bearing. Make sure to measure from the back of the bell to the top of the pressure plate fingers and then add .650 to that measurement for an overall measurement. Call Mcleod (714) 630-2764 X 237 they will tell you what piston you need for your bearing.
The stock OEM Slave in most GTO applications does not stroke far enough to release the disc cleanly. The other advantage to using the Mcleod bearing is that you can fine tune the length of the piston based on your specific application. You can't do that with the stock slave, it is what it is.