I would not use anything like PVC, which isn't flat like a batten. I'd be afraid the diameter would stretch out or rip the seams in the batten pockets, doing more expensive damage to the sail - especially as it flexed in the wind. I think I'd sail lacking the batten before I'd shove in PVC.
You can buy battens online at places like Defender.com, Pyacht.com, etc. The shipping can be a bit steep, since they are oversized. If you have a good supply store nearby, so much the better. They are flat fiberglass strips, about 3/4 inch wide (or larger, depending on your sail). If they don't have your length, get one size larger and just cut it with a saw, like a hacksaw. I also added batten end caps to mine, which are softer rubber caps that help keep the sharp ends of the battens from sawing through the sail. If your one batten is too short, have you checked that all are actually in the right pockets? Maybe that one belongs in a shorter pocket and you are actually missing a different one?
