Your realistic options depend on your budget, goals and purposes.
If you're doing a full on restore youre probably going to want to send them out to be rebuild. I recommend hitting a couple BIG major antique car shows and talking to a couple companis that take your existing metal hardware and sew new fabric on.
If yours are locking improperly then used may be an option, but, degrees of fraying and color fading are an issue, ESPECIALLY with that color.
If you don't care about having gm logos on the buckles, and, if the metal portions being 'similar but noticeably different' doesn't matter to you, there are companies that make 100% new units.
Used will be cheapest, but, possibly tough to find depending on your quality level. To have yours restored/rewebbed will be similar cost to new, but, have a waiting time of usually 4-8 weeks, possibly more now with the virus issues.