I have started buying "used" batteries at Autozone, Advance, and I haven't yet, but the interstate guy at work says he has them also. They are not exactly used, they are batteries that have sat on the shelf too long. I haven't bought any lately, but I was getting them for about $50 each with a 30 day warranty.
I have had very good service so far from these batteries. You have to be flexible, when you go to the store their inventory will vary. I can sometimes retro fit different batteries by turning them around. Two years ago they had two of the same type batteries at Advance, they sit them on the end of the counter. They wanted $70 each. I told the guy I could get them at Autozone for $50 so he discounted them to that. They are a little bit smaller size physically than the original batteries in my diesel, but they have been starting it fine this past winter. I don't plug it in.
I was asking the interstate guy about it, and he admitted the "used" batteries are all he runs in his personal vehicles.