Replacement Catalyzer for Jon-e hand warmer?

I picked up this old refillable hand warmer from a yard sale. With no idea how much life the catalyzer has left in it, I want to pick up some extra. However this appears to be some sort of cloth rolled up in the spring, and all I can find are these little pads, either gray and yellow-ish. Do I just get the pad and slide it in the spring, or is there some other material I need to look for? Any help would be appreciated!