I considered buying the motors from Sherline, but they want $65 apiece for their stepper motors. I knew I wanted NEMA 23 size motors, with reasonable torque, voltage, and current ratings, and dual shafts (the mill handwheels attach to the other shaft). So I checked the usual suppliers, and ended up buying a set of three from a seller on eBay.
I went to put together my mill, only to realize that the "CNC ready" version didn't support manual operation without having dual-shaft motors installed. Oops!