There is an occasional online Mopar shop that will sell you a whole used OEM set, prices vary from $600-$800. Otherwise, you can buy pieces off epay, when they come up, and eventually build up a whole set. I've done this, once or twice, as needs arise. Grab a Dealer Parts Book, and use the part numbers to verify you're getting the right belt. Some epay sellers either out and out lie about what belt they are selling, or they're just unwilling to look up the numbers.
As you (and others) may know, the factory shoulder belt doesn't retract, so once you're strapped in, you may not be able to reach the radio controls, or the glove box, without unbuckling the shoulder strap. I've grown to dislike how the shoulder strap holds me to the seat.
I don’t have a full set but I do have parts available.
The 2 front retractors are both lefts but I do have an earlier right.
The second pic are the shoulder belts.
There are free PDF ones. But a hard copy is fun, too. I have a hard copy of the 1970-71 Dealer Parts Book, but I never use it. I go to the PDF version, because I can search the text.
One vendor option for factory-style restored seat belts...Ssnake-Oyl Products.
Also, aftermarket shoulder belts are available. I used the Wesco belts. Info is in this thread.