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.
If you're not dead set on OEM, there are some nice options that look pretty good, and give you the three point shoulder setup. Refer to post #14 of this thread:
Can anyone help with 70 Challenger seatbelt installation? Pictures?? Measurements?? Please!!
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.