How to produce digital pulses using 555 timer IC for stepper motor?
There is a lot more to controlling a stepper motor than simply producing the phase pulses.  You have to consider accelleration, decelleration, maximum speed, and producing the proper overlapping coil phases for the direction you want to go.  Once you have the pulses, these still need to be amplified to drive the actual coils.

Controlling a stepper motor is a great job for a microcontroller.  This can perform all the logic described above, plus can interface with the rest of the system to be told what the stepper motor is supposed to do.  Since the micro will only put out digital logic signals, no different from the 555 timer, you will need power electronics to drive the coils.  Depending on how the coils are wired, this can be a set of low side drivers or possibly H bridges.  Low side drivers can be done with discrete transistors pretty easily, especially at low voltage.  There are H bridge driver chips available that take digital logic control inputs and drive the coil lines accordingly.