I found this:
I make my own. The best acid solution to use in my experience would be nitric acid with ammonium monobasic phosphate. I prepare a solution with 1-1.5% nitric acid and about 0.2-3% ammonium monobasic phosphate to help decrease the pH of my solution. The ammonium ions also create a pH buffering effect through ion absorption effects that helps maintain your pH levels stable. The added phosphorous also creates a buffer at 6.5-7 and helps your
plants if they are within the flowering stage. Using this adjustment solution makes pH swings decrease with time as ammonium starts to "fight" the pH changing effect of anion absorption.
Other acids such as citric and acetic acid could be used but remember that they will eventually start to cause problems if you have to do routine adjustments. Citric acid creates a buffering effect (however towards pH decreases) and may start to precipiate calcium if used extensively. Acetic acid will create a buffer around 4.5 and it may cause problems due to its physiological effect on plants at higher concentrations.
My advice would be to use my nitric acid - ammonium monobasic phosphate solution, you will see that it helps maintain pH levels stable and you will have to use it less often as your
plants grow )
I hope this helps.
I used white vinegar once w very bad results. I know of others that have used it in a pinch with less dire results. I don't know. I will never use it again. I have seen plants looking very healthy and practically getting bigger before my eyes while the roots are swimming around in 8.5 broth. so putting stuff I don't know about is not going to happen. The white vinegar turned into jellyfish and killed my plant. I'm intrigued with the sulfuric acid idea. I wish I could ask Fatman.