I have a 12 X 3 trapezoidal tap if you want to make your own out of delrin/acetal. I can post it to you - if post it back when you're finished,
As for motor couplings - why not try dumpsterCNC - I am using their trapezoidal 12X3 couplings which connect to a 6.35mm motor shaft.
Get bigger bearings with a 12mm bore to support your lead screw (I may even have some of those spare in the garage - if not they are pretty cheap)
Nathan