After a lot of internet searching I found the Lowe's has a 2 lb can of Roebic Crystal drain cleaner (IIRC) for $8.
You might be able to get battery acid still from NAPA.
You can also get weaker sulfuric acid from sodium bisulfate, aka pool pH decreaser. $6 for 3 lbs, typically. I use it for stop bath. And the pool.