if you are knowledgeable with electronics you can make one yourself using resistors and such from radio shack. a very cheap method if you know what you are doing.
greddy sells a speed cut killer device that is like 3 wires and its around $100 i think. kinda expensive but its like a plug and play thing or maybe just splice in.
its not a mod worth doing unless you will be doing over 115mph often, which isnt legal except at the track. id spend money on other mods to get you those 115mph traps before wasting money on the SCK.
ps - enthalpy made his own i believe, perhaps he can give you a diagram.