I get whitetail back sinew from a local game processor each year, and am willing to offer some of my rapidly growing stash of sinew for trade. I meticulously scrape the meat, fat and membrane from both sides of the tendon, then soak them in several changes of warm water with a little dish soap in it to clean out grease and blood. Once the water remains clear after an hour of soaking I lay the tendons out flat on large aluminum baking sheets and dry them at room temperature overnight. The result of this process are long, beautifully clean tendons great for making thread, backing bows, tying on arrowheads, spearheads, fletches, etc. I'd love to trade some of this sinew for really nice bow staves, hardwood arrow shafting, stone arrowheads, trade points, braintan leather, or other things. PM me if you'd like to talk trade.
Below you'll see a photograph of several cleaned back tendons (top) and a close-up of a couple of them to show how clean and translucent they are (bottom). All of the tendons I am trading are at least 15 inches long. I generally trade by the ounce, because individual tendons can vary quite a bit in size. It usually takes about 10 clean, dry tendons to make one ounce. I have plenty of past trading partners who will attest to the quality of the sinew I offer. I'm sure they'll chime in below.
(http://ericgarza.info/wp-content/uploads/2015/11/Sinew.jpg)