[mattwalt]

I use a sinker mould.

There are a few slingshot ammo moulds on eBay UK - and they aren't that badly priced.

 

[mattwalt]

This is about as cheap as I have seen.

https://www.ebay.co.uk/itm/10-MM-LEAD-ball-AMMO-MAKER-MOULD-CATAPULT-SLINGSHOT-HUNTING-SHOOTING-MOLD/233338523352?hash=item36540f4ed8:g:S4gAAOSwV2ZZlYOJ

 

[mattwalt]

You could just drill a 9mm hole in a plank of wood and cast in there...