Ron, Masking tape can be used to make seat belts, and it has some obvious advantages: It's cheap and plentiful, always on hand, and approximately matches the proper scale in thickness. Be sure to paint both sides (and the edges after it's been cut to size), and make a big batch of material to use, so you can have lots of stock to cut from. It may take a few tries at each belt to get the width just right. Use a straight edge and a #11 Exacto for your cuts after the tape has been painted. I don't prime before appying the color coat, as I've found the tape to become too thick and inflexible after too many color coats (it'll take at least two) if primer has been applied. Also, I presume you plan on using scratch-made or photo-etched hardware, and the tape will quickly become too thick to fit through the buckles, etc.
An alternate is fabric from your local shop. Look for one with the correct thickness and weave size and pattern.