Someone on a thread used a bicycle inner tube. I've been using one to spot lots of muscle-ups and straight front levers and assisted crosses.
Having used bands before, I think the bands work better and they are longer. However, the tube had a hole in it, so it was cheap. In fact if you were to go to a bike store just ask for a flat one and they might give you one ( and look at you weird ). Then again, they might patch them.
They also tend to slip and snap at you.
I was only going to buy the smallest weight band offered and maybe the one up from that. Any more of a spot, and you're probably wasting your time and should be scaling down the skill and just playing around rather than training seriously.