I just use plain typing paper cut into any length or width I need. It can be bent, folded, folded onto itself, glued, sanded, or painted. Once you get it into the proper position, you can give it a coat of thin cyanoacrylate and it will last forever, and can be further sanded or painted to your needs. Painted up with leather brown, it will look like the real thing. You can mark it up with a sharp pencil to represent perforations for buckles and snaps. I've also used it for US canvas belts and web gear by dampening it, pressing it onto the figure, letting it dry a day or so, then cyanoacrylating it. Then carefully using the tip of a knife, carving grooves to represent the "ribbing", and a very fine drill to represent the eyelets found in US web gear. No special materials needed, other than a sharp Xacto blade and ruler and some thin super glue.