We had a similar problem with our Chevy Conversion Van, where the retractor broke and therefore the seatbelt would not pull tight when buckled and was essentially useless as a safety device.
We went to the local Chevy dealer who told us that since the van was more than 10 years old, Chevy no longer supplied parts and it couldn't be fixed. He was certainly not a lot of help.
We found a replacement belt online from JC Whitney or some other company and ordered it. We took it to several local shops and nobody would install it, as they said their insurance companies would not allow them to install these belts or they might be liable if an accident was to occur.
We finally found a local guy who did the install with one caveat: He would not install the very top bolt that went above the driver's left shoulder and held the shoulder strap. He showed us what to do and said that if we put that bolt in and tightened it, then we installed it and could not hold him liable. That worked fine for us so we did it.
Moral of the story: It ain't as easy as it may seem.
You are illegal if you drive without a seat belt, but Chevy won't supply one over 10 years old, and shops won't install one even if you manage to find one.