The hardware stores will almost all have Muriatic Acid and that's what it will be called. It may be in their stone/concrete section since the acid is often used to etch/wash stone/concrete. Sometimes it's in the paint section. In fact, they sometimes have it in both the stone section and the pool supply section, but charge more in the pool section or offer a half-strength (15%) version in the pool section for the same price so it's really twice as expensive relative to its concentration.

The full-strength version will say something like "31.45% Hydrochloric Acid" on the ingredients (the rest is just water).