The proper stepper controllers do not act as a resistive load. They are more like switchmode power supplies. So you are on the right track by calculating it as power instead of current.
The datasheet specs highlighted by Tom will give you want you need. The DC resistance of the stepper coil is the useful number, along with the max current. Multiply by 2 as there's 2 coils in the stepper but they are never both on at once so multiply by √2 instead (about 1.4).
Then your 12V supply needs to give that amount of power to the stepper driver, plus a little more.