I bought pumps that screw on top of all my gallon containers. One pump = 1 oz.
I cut them a little to fit on smaller size bottles.
The openings are the same so it fits on all sizes.
Here is what it says about them where I bought them:
Bottles are made of high density polyethylene. Smaller bottles do not have a handle. The 1 gallon jug pump delivers 1 oz. per stroke, plunger locks down for shipping and storage, ounce and milliliter graduations on plunger, 11" dip tube, color white, polypropylene with stainless steel spring. Excellent for soaps, lotions, chemical concentrates.