You need a separate power supply for each required voltage, and each supply must provide at least as much current as required by the devices (motors...) connected to it.
Stepper motors typically are used with higher than their nominal (minimum) voltage, depending on the required motor speed and torque. Use a separate power supply for the motors, and connect it directly to the driver board. The driver must limit the motor current when the stepper motor halts or turns slowly, that's why you should get a modern dedicated stepper motor driver board, the old L293 is really useful only with ordinary DC motors.