Keith has been playing about with a new cheap power supply using the Nerf Gun motors as a test bed. It turns out power supplies are quite a good method for determining how much current they draw and thus what current the motor driver needs to deal with. Keith produced a quick video demonstrating the power supply with examples using the Nerf Gun motors.
If you are still deciding on motor drivers he recommends getting the motors first and doing a simple stall test with a power supply or multimeter and then getting the motor driver to suit.
See his video here: