The motor shaft of tetrix motors is 6mm, which is ~.236"
(Taken from this drawing)
You can get 6mm shaft from McMaster and a number of other sources.
Of course this drawing of the hub shows the bore to be 5.95mm or .234"
I wonder if you could take the motor hubs and drill them out to 1/4" That would give you a bunch of options. The little nub that centers it inside the gear/wheel/etc. is 7.97mm or .313" so there would be material left to center the hub and use them as normal. D profile shaft 1/4" diameter at McMaster is ~$9 for a foot long piece.