Battens in my early days of sailing were made from cane, that swelled and twisted if not dried out properly, then fibreglass became popular.
People have often used plastic strips but these would difficult to shape.
In the days of Manta / freeflight yachts flat electrical condute was used (which is no longer made...)
I have thought of using the plastic strips used for joining sheet flooring often called blue or yellow tongue but I dont know if it would be stiff enough.
I have some personal views on the use, tension and stiffness on battens which are often apposed to the conventional thoughts, but to hard to explain on the forum, its more a practical thing.

Back in the "good old days" we made battens from damned near anything. Any longer grain timber is ok, play with the thickness, probably about 35mm by 6mm to start with. The old brown formica type stuff works great, for that you make it narrower, rather than thinner to get the foil shape you are looking for. But as Landyacht says, old windsurf sails are cheap enough to score just for battens.