Lowes sell this product:
Basement Watchdog 6 Quart Battery Acid
It is sulfuric acid and will likely work for you. It's about $19. I'm sure you won't need 6 quarts, but sulfuric acid will not degrade over time unless it has something to dissolve.
Safety corner: It makes sense to wear a rubber apron, long sleeves, a face mask, and rubber gloves when handling concentrated acids. Make sure you have a running water hose nearby in case you need to dilute the acid quickly. It can have a nozzle on it, but have it ready to turn on in an instant. And do not pour water into acid. It will create heat much faster than pouring acid into water, and can lead to glass breakage and a big spill. The heat will create dangerous fumes, too. I know that you will not mix water and acid in the battery, but you might use the leftover acid for something else.