Hebert guns has streamlight batteries for about $2 each. A few months ago spanky and i split a box of 70 something from Surefire, the savings were pretty decent.
I don't mess around with cheap foreign 123 batteries they sell on DealExtreme. Those things can turn into little hand grenades. Spend the extra pocket change and get American brand batteries!
There's a dude on the AR15 EE I think his name is calvin and his screen name is brightflashlights that usually has really good deals on them if I am not mistaken.
Brightlights is a good source for bateries, as well as weapon lights. He can also build you semicustom lamps that throw some serious lumens at very reasonable prices.