I saw those but the reason i didnt go with those is because the McMaster flexible coupling is around $40 so i'm trying to find a lower price.
I am using a flexible beam coupling so that any mis alignment between the motor axle and the lead screw casued by mounting it slightly off.
Good idea on the 1/2 to 1/4 coupling
I didnt realize those would be easier to find than the 8mm to 1/4 coupling
I found one from Michigan that is only $13
http://www.cadcamcadcam.com/shaftcoupling-2-1.aspx