The obvious answer is a stronger acid.
I recently attempted to get a bottle of 1N hydrochloric acid that I use for work (about a dropper bottle full lasts a year or more). The chemical company refused to sell it to me, because I asked to have it sent to my house rather than a business address.
If you can get nitric, phosphoric or hydrochloric from a chemical company, it will be cheaper in the long run, but it will cost up front. Also, all that safety stuff that @Pushrod Monkey said.
Sulfuric is a little nastier, because it is a strong oxidant as well as strong acid. That's why any clothing that gets battery acid on it will get holes in the shape of the droplet of acid that hit it.
I agree with the grow shop guy. It's best to avoid adding sodium.