We made a gearbox like that last year using two aluminum plates, with a few holes punched (using a rotex punch) and reamed to size to mount flanged bearings. The shafts were 3/8" hex stock with the ends turned round to fit into the bearings, also one had the output shaft sticking out a ways. We used two of the same gear from the FP transmission, I think it was the second gear in the gear train? it was the white one in the picture above. We swiped one out of another older FP transmission.
As Art says, you can make a gearbox yourself if you just have the guts to try
you could even make it from hardwood such as oak, and use a Forstner bit to make the bearing holes.