bigfootcomputers.com sells the non conductive ones.
somepeople say they trap heat and are a waste of money but they can protect you from breaking your cpu when you install the hsf. I have one on my cpu and If you have hsf fan that goes on real tight or are nervous about cracking the core I'd say for the 15 bucks canadian it costs its better than paying for a new cpu.
if you have installed hsf's before and know you will have no problem putting it on properly you probably wont need to get one.