I know you can get some 6mm round rod on Amazon, (double check that it is 6mm though should be able to see what bearings go in the trans for size) and then use a vice on a drill press, you would want to get non- hardened then harden once you have the almost finished product. Get a good enough...
i have been keeping my eye out, haven't found any yet though.
might be able to custom make one? iirc that shaft is 6mm diameter might be able to use a drill press or machine some holes in a solid rod and make some spacers. still going to keep my eye out though, wish i had been able to stock up...