I just bought some from truckandrv.com. They were new and snapped into the blazer seat I used. They were $35 each, but did not include mounting plates for under the floor.
I have also used ones from southern rods (cant' remember the web site). Lastly, I was looking real hard at the ones from
http://www.andoauto.com/seat_belts.htm
I really like the aviator style flip up chrome latches.
OCB