Sinker moulds are sold in good tackle stores. This is a small selection of the sinker moulds available.
Fishing Sinkers – How to make your own lead sinkers – Video

Pouring your own lead fishing sinkers and jigs How to make your own fishing sinkers.

A variety of heavy surfcasting sinkers made by cutting and shaping a heavy Puka Bomb (shown bottom left).
Heavy Surfcasting Sinkers – Making Extra Heavy Surf Sinkers

Heavy Surfcasting Sinkers – Making a Few Extra Heavy Surfcasting Sinkers for Use in Rough.