Chelated iron can be bought in several forms: granules and powders. Granules and powders are water-soluble and can be mixed into water to create foliar sprays. Spray these liberally on the foliage of the entire affected plant. I also seen a guy juice some spinach. He used the juice with water and fed his plants. Gl
Your best bet is to get a nute that contains all essential micros, like Flora Micro from GH. You can buy iron chelate by itself, but you'd be better off getting something a little more evolved.