Second hand forklift batteries are a cheap source for lots of storage capacity. They tend to be deprecated well before the end of their service life simply because they can no longer make it to the end of a shift without recharging.

Do test the cells individually with an acid tester, a simple hydrometer one is easy to take along when you go shopping. One important thing to keep in mind when buying any kind of battery is that for some batteries there are pretty strict requirements as to how they can be installed, what kind of physical isolation you have to use and how you need to vent any gases that they produce. These rules can easily disqualify some battery chemistries or configurations from consideration.