usually where there is water there are smooth stones and clay, try to find something.
next is to transfer stones to the place.
or buy clay and make clay projectiles .
I want to make clay projectiles with C_As rollers but can't find cheap clay supplier.
If you use ~10kg-20kg of ammo per session that is a problem.
That's why i stick to stones ,no buying, no making , no transfer.
another idea in the back of my mind for years is , to put angled stones(like railroad rocks) in a machine to smooth and round them
i found this:
https://www.youtube.com/watch?v=Qn0jYqrONPw(If you know another method please share)
so if you have your own property but no smooth stones you could make a huge rock tumbler powered by water/air/electricity.
and another thing you could try is use a triangular pouch sling/Y sling so there is little friction on release and little spin rate(now that I think about it I should test Y-sling spin rates with tennis balls to compare to normal)