You can make your own with some dental floss and a little bees wax - like a candle stub. Just pull out as much as you need, and then run it over the candle stub to "wax" it some more.
The imitation sinew is just a version of nylon thread that hasn't been twisted like normal thread. And dental floss is the same. The only difference would be how heavy/thick it is.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands