Originally Posted by Enes Kolenovic
Obviously any substitute will never be as good as the original, but does it get the job done?
In my opinion, at a cost of less than $15, it's hard to complain. Plus, if your erectors are fried and tightening from DLs, cleans, GHD back extensions, it feels good to lie stomach-down on the ball to stretch your lower back.
The appropriate ball size is usually recommended based on your height. (E.g., I'm 5'6" and I use something which is supposed to be "65 cm".) I think if you go too big you might lose a little range of motion.