Casting zinc has the same pleasures as casting bullets.
Only more so. Zinc shrinks more.
I had a mould made locally (4.5"). They chose sand casting, where I'd asked for an aluminum mould. Didn't work out. You need good pressure (especially as it starts to cool) to keep voids from occurring. Mine turned out oversize and out of round.
You will have better luck to have a commerical foundery (as listed by DD) do the work.
If you have voids there is greater potential for the ball to not fly true and may crack more easily.