I ran into this same problem a few years ago. No one will sell you a replacement seatbelt for safety reasons, other than the dealer. I tried a bunch of junk yards and all told me no.
My dealer wanted $350 for the sealbelt, 600 installed and then a 10% discount on top so I just had them do it; figuring it wasn't worth playing with all the air bags over there.
Thor - 2004 XC70 (225k and going strong!)
Gunnar - 2013 C30 T5 - Traded in on...
Ty - 2013 S60 R-Design!!
Yeah, I guess I'm a Volvo guy... ;-)