I would first try running them from 1 AA battery per and see if they light and are bright enough. If they are, then you are done. If not, then you need to know how much current the LED's are rated for. Once you have that info, you can use any of the online LED resistor calculators to figure out which resistor you need for them.
On the other side of the coin, however, why not just run a power line from the encoder you are connecting the buttons to? Batteries are no fun, especially if you can not need to worry about them by running an extra wire.
RandyT