If you have the old one, you should be able to measure and come close to the correct size. If you have some calipers, then measure the inside diameter of the groove it sits in. Then the width of the groove. And compare to the same measurements of the old O-ring itself.

Not sure why Ace didn't have a size to fit. But try auto parts stores or browse the world wide web. Although it might be the cylinder walls are worn or some light grease on the ring and cylinder walls or something else is needed also if you actually tried some you thought might fit.

If this is a unique shape and not a round or square cross section, then there might be more to it.

My 2¢. Buy a new pump and be done with it. Fiddling with stuff takes time you could be out cycling.