Check out various 3D printer suppliers. One has a coupler that is close to what you need:
http://www.makerfarm.com/index.php/h...z-coupler.html
is is 5m to 8mm.
Also check AmazonSupply.
The other thing I would ask is how much torque and speed to you need?
Lots of 3D printers use heat shrink or plastic tubing for low speed/torque
applications.