There's probably a LOT of content on this site I just haven't poked into yet, and that must be one example. Thanks, and I will check that out.
In the meantime, I found a local source of golf balls that I will call on the telephone later today. It's a Boys and Girls Club that is fundraising by selling old range balls, and their Craigslist listing said SEVEN CENTS PER BALL, OR BEST OFFER, and they claim to have 20,000 in stock!
The listing was put up in mid September, so hopefully they've still got them.
At 7 cents per, I might buy myself about 400 or so. I've been enjoying golf balls as ammo. Now, there's a good chance that if concrete balls are more massive for the same size, I'll like them even more, but they'll probably tear up my sling, won't they? (The cradle is just woven paracord.)