I get mine from BJs. A box of 12 goes for $10 when they're on sale. I also change them out every 2 months (the old ones usually have enough charge to make it a year in the smoke detectors, so it's a win all around!)
Please do not use used batteries in your smoke detectors. Also, use good batteries in smoke detectors (I use cheap rubbish in my basses but spend up buying for smoke detectors).
I would prefer to have a bass fail on stage mid gig than to have a smoke detector fail.