You get cannonballs in OSRS by making them yourself : after partially completing the Dwarf Cannon quest and having 35 Smithing, use a steel bar on a furnace with an ammo mould in your inventory. Each steel bar makes 4 cannonballs, and Edgeville is the most convenient furnace for banking them quickly.

Best places

  • Edgeville Furnace: easiest overall because it is very close to a bank.
  • Port Phasmatys: another strong option if you have the access and route for it.
  • Al Kharid: workable, but usually not as convenient as Edgeville.

What you need

  • Partial completion of the Dwarf Cannon quest.
  • 35 Smithing.
  • An ammo mould.
  • Steel bars.

Quick method

  1. Bank steel bars and your ammo mould.
  2. Go to a furnace, ideally Edgeville.
  1. Use the steel bar on the furnace.
  2. Choose cannonballs and smelt in batches.

If you lost your ammo mould, Nulodion can sell another one for 5 coins.

TL;DR: Make them at a furnace, best at Edgeville, using steel bars + ammo mould after Dwarf Cannon and 35 Smithing.