I just made some out of some nylon webbing that I had around. Cut it to roughly 18", put a loop on one end, slide the other end through the loop and you're ready to rock. Otherwise, check out IronMind. Hope this helps you out.
I made the mistake of buying some from SportsAuthority. They didn't work well and Ben told me to just make my own and that those tended to slip (which they did on pulls). I finally did about a month ago and have been using them ever since. Way better.
I made mine about a foot long but I should have gone a bit longer. Still work great.