Make my own.
Sorry had to add that one in here since no one has mentioned it.
They are real easy to make with a section of old 2-1/2" leather belt, 3/8" rubber tubing, & 3/8" Braided Nylon rope w/ working strength of 250LB. or better.
Take the leather belt & cut a 4" section.
fold the section of belt in half & sew it on the sides making a pocket.
Drill 2 holes in the pocket on each side of the opening.
Thread the nylon rope through the holes & tie it off to make the bottom cup of the stringer.
Here's the tricky part, cut off enough rope to make the stringer with enough room to have the rubber tubing loop.
Thread the free end of the nylon rope through the 3/8" ruber tubing "tubing should be about 3" in length, unless the limbs are narrower then a smaller piece will work."
Once the rope is through the tubing make a loop & tie it off so you now have the top half of the stringer.
Once made all you have to to is adjust the length to suit your needs by untieing & adjusting either end & resecuring, but it will last for many years.