I need to buy/rent/fabricate an ST-125 spark tester to diagnose my sisters '94 3.1L Grand Am. It is a special tester used to determine if there is enough voltage to jump a large gap in a waste spark system (two cylinders firing at the same time) distributorless ignition. I can't find it on the web, anyone know?

You can pick one up from one of the local tool distributors (Snap-on, Mac, Matco....the big tool trucks you see driving around)

You can make one with an old spark plug, cut off the electrode and add a ground wire

what kind of gap should I make it with?