There is a company called Ssnake-Oyl Products that is a premeir seat belt restoration business. They make original style seat belts for any car, color, etc. You can have your seat belts restored to coucours condition or rewebbed, or in your case, order rear belts. Their work is awesome, the company started with a Mustang guy who restored a Mustang and couldn't find quality seatbelts, so he bought the webbing and tore his down and restored them. It grew from there. A friend in my Mustang Club owns it now and they have expanded their line of products. Check them out online at
http://www.ssnake-oyl.com.