The Gung-Ho Box is easy to build from hardboard and duct tape, but to me, using it as a vanish device in Strat-o-Sphere always seemed like putting a round peg into a square hole.
Jim Gerrish came up with two better methods of vanishing the Pit Ball, which he includes in his e-Book "Balls to the Walls" which is part of The Wizards' Journal #11. Either one of his methods makes more sense than putting the ball into a cubical box.
But if it is something you feel you really need to use, good luck with the search, or make one yourself as a weekend project. A quick improvement you might want to build in is to NOT use the Gung-Ho arrangement of doors, but the Jack Gwynne Flip-Over Vanish arrangement of doors. Drop the ball in and flip the doors open quickly using just one center hole drilled in the bottom of the box. It makes it look as if you are flipping the box around your finger almost as soon as the ball enters the box... and it's gone. To insure the ball doesn't fly out, try neodymium magnets- one in the ball and one in the door of the box that you want it to adhere to instantly.