I was at Walmart a couple days ago and saw some straps that may work as tree straps. I didn't open the box to see what the material was like, but each box had two lengths of 18" webbing with loops sewn into the ends. The working load was over 500lbs but I don't remember the exact number...so right on the edge of what we'd consider safe. And they're only $2 or so, IIRC.
They're in the automotive section with the tie-down webbing...who's gonna test them out?!