unfortunately the guy who was advertising a repair service on
has ceased so you will have to shop around or buy a complete new mechanism. Be sure which one you need and check to see if the old one is a 6 pin or 2 pin type. Then look on
for the cheapest option, some are sold without the motor (which can be swapped anyway). Replacing the mech is explained in the forum so have a look for the details, it usually takes about an hour.