I've been puzzled by this whole thread, given that the subject line say properties similar to spring steel. I don't know much about material properties but I question whether any of the suggestions made so far meet that. Certainly I don't think anything made with wood would do that.
Bows for hunting has been made out of wood for thousands of years. And if they did not spring back they will not function...
Modern compound bow limbs are made from glass fiber / epoxy composites and a lot of leg prosthetics like the ones pictured above is made from carbon fiber / epoxy composites.
But for ease of manufacture and low cost high volume production i wold think that a fiber reinforced thermoplastic will be a better option. But i dont know if it has the same properties as the fiber reinforced epoxy materials mentioned above.