I have recently added jump seats to my van.  I was lucky enough to have
original seat belts to go with them.  However, you can use any standard belt
that can be obtained from any vehicle or new from your FLAPS.

The essential difference is that the non-original belts likely will be
floppy when not in use, unless you get a roll-up retractor for one side and
a stiffening sleeve for the other side.

Budd Premack
86 Syncro, 84 GL, 73 Super
Minneapolis, MN (Land of Frozen Waters)