If by unable to buy at the store, you mean that you can't find it, then look in a hardware store around the paints. Acetone is sold as a paint thinner. Some nail polishes (fewer and fewer now) are just watered down acetone. If the acetone is just watered down, then you could try distillation. Check wikipedia for boiling point of acetone. I'm sure it's well below that of water. Remember if you do try to distill it, acetone is VERY FLAMABLE. I burnt some arm hairs of with it once.
What ingredients are in the nail polish remover other than acetone?