I agree with David I've only ever been asked to make a couple of balls, but I think if it was a more regular thing or a large batch was requested I think that a jig could be a good solution for consistency and to save time.
I did watch a YouTube video where a guy actually made a jig out of reinforced nylon/resin on his lathe to turn up to 8" balls, I will try to find it out as he did include plans to the one he made, might be of some interest to you. For more regular/daily use though I would suggest making it out of steel or getting a quote from convac in Ellistown to commission one. If this is of any interest for you let me know and I'll find out what I can to help.
All the best.