line6 don't supply internal parts to anyone but service centers, so you would have to go through a service center and see if the are willing to order this for you.

apart from that, the only option is to buy the whole dock.  but you could always sell the spare module you now have on ebay, and also the old dock, as someone somewhere may have worn out the actual pedal or something, and have their own working circuitboard, etc.

Cheers

Andy