Where do you get them..? Graupner's regular line of sport props that were carried at local hobby stores were always more expensive and I almost never bought them just for that reason.
I'm fairly certain this plane and engine will respond to props with less than 6 inches of diameter. I'd like to see 5.5 pitch multiplied by 30,000 as a workable goal.
With the big crank threads / big spinner nut it limits the possibilities just a little. Some of the carbon props that showed up might not have enough meat at the hub..but I'm not sure yet.