I bought a set of black belts with chrome buckles from ebay seller seatbelts-r-us. They are not exactly like originals but they are acceptable for me. The metal parts are actually made in Oklahoma! Great pricing at $58 per pair shipped (watch the ship charge on single orders as they combine). Sumitt also had some decent prices on Generic belts if I remember.
If you need reproductions or yours rebuilt you can try ssnake-oyl. Looks like good stuff but much more $$$.
I might get lynched for this but...on parts that are not mopar specific, I think of what other parts I can use. Mustang parts have been reproduced for a long time and are relatively cheap. I bought seat belts for my '67 Coronet R/T from a Mustang vendor for $17.00 per seat. All colors, chrome buckle with hardware...can't beat it.
I happen to need the front seatbelts for my Dart and I just yesterday ordered the green ones from jc whittney. They were $25 a pair which seemed fair and they offered them in green which is what I need.They might not be perfect but they should do the job .