I did the same as Krazyglue; drilled a hole in the rod and attached another screw, using the preexisting holes in the arm cuff as a template. I did this because the arm cuff would squeak slightly at the end of each sweep, but never broke. The extra screw/nut stopped the squeaking and makes the arm cuff sturdier.
I tried the after market stainless steel version sold on eBay, too. It's sturdy and well-made, but I thought it made the detector heavy. The White's arm cuff is made of light weight aluminum.
White's sells the arm cuffs on their web site in some accessory rod packages, but should offer them as a single accessory at a lower price. Since they will send you a free one to replace a broken one, all the better.