Try, Sunflower Outdoor Sports , Gun and Surplus or Amazon. And yes they work very will on the older pencil barrel Minis. They help the newer tapered barrel 18" models and marginally on the tapered barrel 16' tactical models. What are the 1st 3 numbers of your serial #.
I have got all mine from Sunflower Sports. They work awesomely. ( is that a word ?) On the older pre 580 series, they work the best if you do the "dimple option". Take out the two small set screws, one at a time, and drill a small dimple in the side of the gas block for the set screw to dig in to. That ties it in to the gas block and adds more rigidity.
Without doing that, your just hanging some weight off the barrel, which helps some, but not as much as doing the dimples.