When Dad made straps for his Scratch built figures, he used a variety of materials. For largeish figures he'd use really thin plasticard. I have seen him use tissue paper. For that he would lay a sheet of tissue on a sheet of glassheld tight and flat. Paint a strip with uho glue longer and wider than you need. Uho is flexible, so once dry, really sharp knife or razorblade cut what you want, fit to what ever and paint.
Buckles, one single strand of copper electric wire. Take a length and wind tightly around something of the diameter you want. Carefully run a knife along to cut into rings. Glue on and paint. Someone's going to say buckles are square, but at a small scale no bugger will notice.