You can use old motorcycle batteries for this project. About 75% of the time people throw away good lead acid batteries that are just behind on their maintenance! In fact after restoring them you may get more output due to the charge plates being already fully formed due to acid exposure during charging. You may want to check out youtube on how people restore them (and dump the acid) and use the method you think is best. Good search criteria is: alum battery conversion, or lead battery restore. After restoring the battery with the acid in it, just remove the acid, and substitute the final electrolyte with the epsom salt/distilled water mixture.

But remember when dealing with the old, now useless acid...it can burn you.... and lead is poisonous.

Notice alot of these videos teach you how to dispose of byproducts properly and handle it; such as wash your hands, or any surface your hands might touch with soap and water when your finished dealing with lead(found on the terminals and in the acid). Also wear gloves, old long sleeve shirts, pants, and eyewear when dealing with acid. Wash any part of your body that comes into contact with it immediately or it will burn over time. Dispose of it properly in a clean sealable plastic painters bucket ( call local hazmats at your garbage dump and/or battery recycling place, usually a free service). Not all encompassing but it covers most of the hazards.