Muriatic acid is usually in one gallon jugs or a box with two 1gallon jugs.
Kem-Tek may be a brand name you can find. But any muriatic acid will work.
Chances are you will find it in 31.45% or 14.5%.
A hardware store, HD, Lowes, Menards, TruValue, Ace, whatever should also have it.
Plain...