You can get any kind of leather you need at Tandy leather online - buy the material, cut yourself a new tail strap (the adjustable extension part), dye it and add a finish coat to keep the dye from coming off on your clothes. You may need to learn some technique to cut the adjustment slots into the material. I'd probably use a hole punch at each end of each slot and a chisel to cut straight lines between the hole punches.
You won't spend almost as much as a single strap costs for the materials, and you'll have plenty of material to do multiple straps. Their Tan-Kote finish would work well on guitar straps. http://www.tandyleatherfactory.com/e...-2220-120.aspx