High Seas: Don't expect much RPM from them. Most of them are rated for 12V. That means to get a "high" speed you would be looking for up to 300V drive!! Of course I don't suggest you try that, as the insulation rating is probably far from that. I'm talking chopper drive of course.
So just go as high as you can in voltage and live with the speed you get. The best ones are the older that were almost a cube shape
http://www.metallhobby.net/Lopper/DSCN1761.JPG . The later round-flat type
http://www.metallhobby.net/Lopper/DSCN1761.JPG are less desirable. They often had sintered bearings too, not ball bearings like the older ones. I guess they found out the stepper was not the component that got worn out, so they lowered the specs on it.