Take a look at adafruit's motorshield (Adafruit Motor/Stepper/Servo Shield for Arduino kit [v1.2] : ID 81 : $19.50 : Adafruit Industries, Unique & fun DIY electronics and kits), it can control 4 motors.
And there's a tutorial (Method for stacking two Motor Shields to control 4 steppers - adafruit industries) on how to use two of those at the same time.