I was going to get picky with my water profile on my next batch, and was going to pick an acid to lower my sparge water pH that didn't impart flavors. So I'm avoiding the powered acids that are used to flavor wines (which is all that's available at my LHBS), and I'd like to go with liquid. I'm seeing mention of phosphoric acid available at the hardware store, but I didn't know if that was just for cleaning and rinsing equipment, or if putting a few drops in the sparge water would be ok for human consumption. Or if that's not good, other alternatives available at the hardware or health food, or some other store.
Phosphoric acid itself is consumable, but to make it food safe, processing and packaging have to be done in a food safe manner.
I use lactic acid 88% to acidify my mash and sparge water. Your LHBS probably carries this. Only a few milliliters are required, so a small plastic syringe or pipette is great for getting an appropriate dose.
As well as using acid directly, you can lower the pH of your mash with Calcium Sulphate (Gypsum) and Calcium Chloride. These also affect the flavor profile of the beer, allowing the brewer to bring either the malt or the hops forward.