Nymo nylon thread is commonly used by beaders, myself included and will not break down like cotton. I have also used it in my museum work in the past. I do not recommend silk, sinew, anything intrinsic or fishline. Fishline plastic/nylon biodegrades fairly quickly or can become brittle and break. I would use size D white Nymo thread. You can order it from Crazy Crow Trading Company. You might also consider synthetic sinew but is is waxed…both are available at Crazy Crow or other Indian Craft enterprises. You will probably need some size 10 or 11 beading needles also.