Try doing what Re-design suggested, but if you still can't find the battery, you're going to have to make one and reuse the casing. I'm assuming the battery is a Ni-CD or Ni-HM. Carefully open the battery case and there should be about 16 cells, each giving 1.2 volts (1.2v x 16 = 19.2v). Buy 16 new cells of the same chemistry and equal or higher mAh rating than the old ones. Solder them in series and smush them back in the case. Seal the case and try it out. It is important that the mAh rating of the new cells is equal or higher than the mAh rating of the old cells because if the rating of the new cells is lower, the charger will pump too much current into the new cells and ruin them. Good luck.