The hardest part is finding one with low enough RPM's. Standard motors are 1735, which even with speed reduction via pulleys gets it down to the 300 RPM range. About 300 RPMS at the mill itself is about as high as you want to go.
You can harvest a motor from a variety of applicances, dryers are good. Just make sure you look at the motor RPM's and the wiring diagram from the appliance so you know which wires do what. I got mine from a garage door opener which was rated at 1000 RPM's. The speed reduction gave me about 170 RPM's at the mill which works very well. The problem is, this motor wasn't meant to run 5 minutes at a time, so the thermal cut out kicks in for big grain bills and I have to wait a few minutes for the motor to cool down before I can finish milling. I see a new motor somewhere down the line.