The Studica Robotics D-Shaft Coupling has an ID of 6mm and an OD of 12mm.  It can be used with the 6mm D-Shaft Axle and a 6mm shaft DC motor. 

Precision Hub for Hex-Shaft Couplings

The Studica Robotics 6 mm Low Profile Hex-Shaft Clamping Hub V2 is engineered to deliver a secure and compact coupling between your motor or drive shaft and wheels, sprockets, or other mechanical components. With its 6 mm hex-shaft ID and low profile form, it's ideal for tight-space and high-precision builds.  Made of aluminum, the Studica Robotics Clamping 6mm Hex-Shaft Hub-V2 is used with flex and drive wheels