Janice Arthur makes them for lots of cinematographers: steadijan@hotmail.com
2, 6.5, 11, 15 lb plates or 7", 8.5", 9.5" rods to make a cage.
My school had a cage from her for when we ran DSLR's or smaller cameras on our Archer and Phantom.
MK-V make cages as well, though I'm not really familiar with their product.
http://www.mk-v.com/?p=1149
On the cheap side I've used ankle weights from Dick's or asked machine shops for scraps.
What steadicam/camera combo are you using out of curiosity?