It is very common to use a much higher voltage than the stepper motor rating, because that allows the current to change very quickly, which means that the stepper motor can run faster.
Obviously, a much larger voltage will result in too much current unless something is done to limit the current. The resistors that Crutschow mentions is one way of doing it. A more complicated, but more efficient way is to have a PWM controller. Various ways of doing that are mentioned here:-**broken link removed**