I know some JY's won't sell seatbelts. I don't think they're supposed to. But it's been known to happen. The earlier ones (I had a 1990 do this) are the GM style, but I couldn't swear that you could pull one out of a Chevy.
The other semi-permanent fix is take the center one out of the back seat. In the pre-97 at least, there's one bolt (that's like a T-45 or T-50 Torx) that holds a twin "female" belt buckle socket. Take that one out, put it in for your driver's seat, letting the spare one flop around. That way you still have your co-pilot's seatbelt, and one functioning seatbelt in the back.