I go to the Interstate Battery dealer and buy the "blemished" ones at a good discount
I worked at Interstate part time while in college and remember this. We had a rack in our back room with old (but good) batteries that customers gave us. They all sold for $40 regardless of size and had been fully tested on our Cetek device.
I snagged a group 65 with extra capacity during my last few months working there, and the battery lasted about 4 years (on top of its age at the time we acquired it in the store). Can't complain for $10/year cost.