From my understanding, you can't just buy the interface cord. If you have proof that you bought the sim, Reflex might help you out, but I doubt it. The interface cord is how they keep their copy rights protected.
They will replace any damaged cord if you send it into them, no questions ask, but I don't think they will replace or sell you a new one if it is lost or stolen. You will probably find your going to have to buy a whole new sim if you want the interface cord.
Have a look on their web site, I think they have a big right up on there somewhere about all this.