I've found it at appliance salvage places as oven gasket. Sometimes it free but you'll need a 20 year old stove. They use silicone gaskets on modern ovens.
I've also made my own from tightly rolling glass roving to the right diameter and tying off the ends with wire. My Optimus Nova has a "priming pad" made from several circles of glass cloth (like stacked washers). The center post of the stove passes through the hole in the middle.