I think Chris is right. I saw a short film clip one time on how they are made. If the air bag gets deployed, it will also fire the pre-tensioners which will literally pull you back against the seat. Once they have been fired, you have to replace them. And to replace them, since they are an integral part of the seatbelt mechanism, you have to replace the entire seatbelt.

I wouldn't try and find them in a junkyard either. They are explosive devices, just like the air bags. It doesn't take much to make them fire. Plus, if you replaced them, that is, not by a dealer, how do you think your insurance company would like that?

-G