I use the thermasil ones. you use insulators to get good thermal transfer. The heatsinks 
are isolated from everything, so you don't even need insulators, but if you use thermasil 
or other conforming ones you get better thermal transfer. Even if there is a short between 
the case of the transistor and the heatsink its ok, as long as you don't touch the heatsink 
  
I'm pretty sure this is the one I used mouser part number 
567-175-6-310P