The two most common batteries are the 3.6 volt TL2150/S or Equivalent and the 4.5 volt Rayovac 841. Most Mac dealers can get their hands on the 4.5, and most of them stock the 3.6 (which looks like half an AA battery). Radio Shack also sells this battery because it is used in security systems as well. They sure don't keep it around for us! It's $10 there, but more expensive elsewhere. Wholesale price is about $4.50 from BatteryBiz.com, but you have to buy in quantity.