I just made the aluminum one myself out of 1/4" .  I guess you could purchase one from me or get someone else to make it locally. It likely took a couple of hours. I just made mine because the other one didn't fit .049 motors and I needed something else. I really don't use them much for breaking in like the other guy speedracerntrixie says.  I did test out a whole bunch of motors over the winter just to sort out the slower and faster ones, and to see if some swap meet deals worked and it came in handy.