This motor from Pololu has a 0.275" (7 mm) D-shaped metal output shaft that matches the Pololu wheel 42x19mm and 32x7mm (listed below). The face plate has two mounting holes threaded for M2 screws.
These motors are intended for use at 6V. In general, these kinds of motors can run at voltages above and below this nominal voltage, so they should comfortably operate in the 3 – 9 V range. Lower voltages might not be practical, and higher voltages could start negatively affecting the life of the motor.