ball bearings.
Like my favorite ammo (other than ammo I make) Try dry wall plants. They have shaker mills that break up gypsum for the dry wall The shaker mills use 1 inch dia ball bearings. what used to do. when I lived near one is leave them My card when they trade out the ballbearings because they were rusty or too pitted I would buy them for a little above scrap prices.
Also watch scrap yards they sometimes get them. I leave card with them too. But with my smithing I have a good relation ship with scrap yards.
Speaking of the smithing ill be back in oregon by april. and hammering away again
Sorry adding to this post rather than a second one
My first taste of the power of the sling was though aball bearing
I was maybe 15 feet away from a old paint can full of hardened paint. Well not only did I hit the can But the ball bearing went completly though the can and the solid paint
After that i was sold on ball bearings