For a bit of cross-cultural sharing of knowledge, many American Indians use what we call artificial sinew for heavy duty stitching. Most times, when we sew two pieces of leather together we use artificial sinew. It might be usable for sewing your buckles and straps, which is why I point it out. Its usually already got wax on it though. Some people even use a high quality waxed dental floss. I've heard the Inuits often use dental floss for stitching their mukluks. Not that I expect anyone to go trying to use either of these materials, but if you do, I'd love to know. Although, artificial sinew is excellent stuff to have around. We usually wax it up good with beeswax when stitching leather pieces together. It can also be stripped down into thinner diameters by the user. These days you can even get artificial sinew in various colors. In the old days, real sinew would have been used but this artificial sinew is so cheap and easy to use that real sinew is used by 'purists' trying to keep the old ways alive. Good for them I say.