Potter's wheels generally only come with one bat. If you want to be able to throw multiple pots, you will probably want to have a number of bats on hand. This can get expensive!
Particle board, pressed wood and Masonite bats are the least expensive kind to buy. However, "store-bought" versions will still cost you over six times the amount of money you will spend making them yourself! (Granted, this is if you already own or can borrow the common tools you will need.)
Begin by checking how high your pottery wheel's pins stick up. You should make sure to buy particle board that is at least that thickness (usually 1/4 inch).
Note: You can also use pressed wood, composite board, and Masonite. Do not use plywood! The layers will delaminate due to the water and splinters will become a hazard.
Beside the particle board, you will also need a jigsaw, the pottery wheel's original bat, a drill, a drill bit that is the same diameter as the wheel head's pins, a marker, a ruler or straight-edge, and a hammer and nail.