Not that I recommend it (sealed batteries are best for cars, IMO), but sulfuric acid is readily available at Lowe’s and Home Depot. They both sell marine batteries for use with battery-powered backup sump pump systems. Those are shipped dry and you add the sulfuric acid and water at home. It’s been about 3 years since I last had to buy some, but I want to say it cost around $15-20 for enough to fill a marine battery about the size of most car batteries.
The catch is properly disposing of the excess. It’s hazardous waste, and must be handled properly - which means finding a disposal agency. Here that means saving it until the summer when the county has free drop offs, or paying an agency $50-75 to take it other times of the year.