There are several vendors for "street rod" seat belts. None with retractors, but those can be transferred. Inexpensive enough you can put in the replacements as you ponder how to get the existing ones refurbished.
There used to be a place called "Snake-Oyll Enterprises" in Porter, TX that did rebuilds on OEM seat belts. Not sure if they are still around, but you might look in some of the repro parts section of a Street Rod-oriented magazine or similar. You might do a Google search on "seat belts" to see swhat might come up.
Years ago, a car club friend had a '68 Charger he was doing some work on. He brought a seat belt out of it to show us how well it cleaned up. He said he wrapped the belt in a bath towel, held it together with rubber bands, and put it in the washing machine with Tide detergent. Not sure how the detergent got into all of the areas, but it did. The webbing looked NEW and the chrome was bright again. We were shocked.
As to the buckles, you might be able to clean them with club soda (mentioned in another forum) and then repaint them silver or satin black? Then put some white grease on the pivots inside of the outside buckles?
Just some thjoughts,
CBODY67