Not sure where you can find original shoulder belts. But, to install more modern 3-point belts go to
http://home.pacifier.com/~dnstoys/seatbelt.htm. There, you'll find detailed, illustrated instructions to do it along with a source, price, and phone # for the belts. According to the site, all late 68's and newer models came with factory mounting points for the shoulder belts. However, it was an option - probably only included on the GTs. The hidden, existing upper pivot mount is hidden under the headliner right behind the coat-hook. Hope this helps.