In a pinch I have made some decent screwdrivers from old allen wrenches that I ground to the correct dimensions. If you can grind them without taking out the temper so much the better, if not heat treatment is a simple activity that I have my shop students do all the time. You could also regrind the mild steel screwdrivers and than caseharden them afterwords, though this would be a less desirable method in my opinion.
For a really first class set buy some 1/4 and 3/8" high carbon hex stock and make a complete set, Howe gives very good instructions on this topic.
