use enhanced transistors to drive the motors, like IRF630, and be sure to put diodes in parallel with them to prevent current from the stepper motor from going back into the circuit and frying your circuit.
below is an example of what i mean: (the 1k resistor is just a current limiting resistor from whatever your driver is, might not even be needed), you're gonna have to set up one of these for each wire on the stepper.