Great quality large leg tendons from water buffalo. One tendon weighs on average about 33 grams, and is about 30 cm long, however both length and weight varies a bit. These are great for wrapping both nocks and points on arrows, or for backing wood bows - and of course for making horn/sinew composite bows. It's also possible to make super-cool bowstrings from these!
How to make sinew fibers from these? Wash them lightly with soap, rinse well with water, and let dry for 24 hours. When they are dry, use a mallet/rounded hammer and pound the tendons lightly until they become white and start to separate lengthwise. You can also use two rounded stones to soften them. When possible, split them lengthwise. It's good to try to split them into two equal halfs - that way you get longer fibers. The fibers can be separated into thicker or thinner threads, depending on the use. American-indian wood bows were generally backed with rather thick fibers, while horn/sinew composites are backed with fibers like sewing thread.