fox seat belts

I can't imagine that it would be terribly hard if you could find a donor car in a yard. Basically, you'd need the shoulder harnesses, interior plastics and probably the sheet metal mounting point for the upper mount. I'd guess it would require a little cutting and welding for the upper mount, but it should be doable.
 
If you do fab-in the rear retractors, make sure they are oriented EXACTLY like they were in the donor car. Seat belt retractors have inertial components that make them lock up in a crash, and if you have it oriented wrong, they will be almost useless.

Make sure to post pics if you do tackle this, I'd like to see it done.