Paul, those seatbelt retractors can be bought at some of the OEM parts sites for older GM cars. If yours is like mine, its black with a long extender.
As an upgrade, while visiting the Airstream factory 2 years ago, I complained about lack of support in the seats and fact they were positioned so high I sometimes had to drive with toe of shoe.
Tech suggested I consider replacing both seats with same color seats used in the new Interstate. Besides better support and adjustments, they also come with shoulder belt harness built into the seat. They were running about $500 or less apiece. Take a look at them on the AI site as they have the Airstream logo on them too. Don't believe they were power seats but did have the pneumatic control with seat heater option.