As for the wiring - you'll notice only a single pair of wires come out of the joystick for the buttons, that's because both the trigger and the button on the back are wired together, so while there are 2 buttons, they act like one button. One of the wires is ground and the other one is wired to button 1 (or whatever) on your ipac.
You should get one from Groovy Game Gear, Paradise Arcade Shop, Ultimarc,
this one, or
this one instead. EDIT: The listing for the one
here that Lew linked to earlier says that it is a 3-wire version, but the picture doesn't show the third wire.
- They have 3 wires so the trigger and thumb switches operate separately.
- If you already have one of those two wire triggersticks, you should be able to run three wires through the shaft and cut the jumper between the non-touching switch tabs so it's wired like this one from PAS.
There's also
this one that is a 3-wire, but I'm not sure how the different trigger shape and having the switch on top changes the feel and ergonomics.
Scott