I just recently purchased a clamping kit for my 8520, and got it from Enco. 9/16" slots, 1/2" studs. They have the same set available in 3/8" studs also, but it a bit more expensive. The set I purchased was the import set, for $55.95 model 400-2562
Most of the time I don't need anything quite that heavy, so I am thinking about also purchasing a 3/8" stud set, model 240-7207 but with the higher price tag of $65.95. Still on the fence on that purchase though. I might be better off just getting some finger style clamps for the smaller stuff.