Pretty good info on the Arai is here:
http://www.precisiontandems.com/arai.htm
They're fairly simple and you can sometimes find them up for sale on Ebay. Most LBS who use QBP as a supplier can also order one for you. Your best bet for a turn-key set-up would be to patronize one of the tandem speciality dealers, such as TandemCycleWorks.com (Colorado), TandemsEast.com (New Jersey), TandemsLtd.com (Alabama), GTGTandems.com (New York), PrecisionTandems.com (Kansas), or bike123.com (Maryland).
As for actuating the thing, for folks with STI or Ergo, a bar-end shifter is the preferred method; it allows you to set the drag brake with a variable amount resistance to control your descent, and then move your hands back to your primary brakes to further modulate your speed on the descent, e.g., to work as a drag brake. An MTB thumb shifter also works well for stoker-operated Arai brakes or for tandem captains who use bar-end shifters for, well, shifting gears.
As for other options, I don't believe your frame has IS disc brake mounts so your best bet for an auxiliary brake is the Arai.