This high quality aluminum flexible coupling can be used to connect your stepper motor to your lead screw. The flexibility will help with any misalignment there might be.
Shaft: 1/4" X 8mm
Outer Diameter: 20mm
NOTE: Please make sure that the coupling is tightened all the way down. With diameter tolerences you may find these couplings grab at the end of the adjustment limit. So, really give them a good tightening to ensure they grab.