Amazon has the ready-to-use 4.2-lbs Boise Garden Rich Aluminum Sulfate Powder for $9.59  with free shipping on $35+ or with Prime. 

Aluminum Sulfate helps lower soil pH for acid-loving plants such as Azaleas, Camellia, Gardenia, Hemlock, and Holly. It works in all types of soil including sandy soil, loamy soil, and clay soil.