KVT, the rings are made of grey cast iron which is readily available from most metal/iron suppliers in the USA. They are then heat treaded to provide the amount of spring resistance to the cylinder wall and the end gap clearance set. Finally, I run them over a piece fine emery paper on top of a piece of glass glued to a plywood base to achieve the side clearance in the ring grove.
In my opinion, using Meehanite cast iron would be great if I were building a full size steam engine to be used on a daily basis for some type of heavy work. In my case, a model steam engine will perform for a long time, (beyond my existance), with grey cast. I have also used viton o-rings in the past and they too will work just fine since the steam generated in a model boiler will not be super heated nor will the pressures exceed 100lbs. My wife is into knitting bigtime, so I have used graphite inpregnated yarn on occasions for rings, but doesn't last. I still use some of this for packing gland seals which seems to work out just fine. Since this is a hobby, anything that you can dream up to try is worth the effort, after all, isn't this how we got to where we are today.
Regards