There seem to be no javelins in any shop which I have only noticed after receiving a Heavy Ballista drop. This also explains the weapon is pretty useless if demonics and dragon imps are the only way to get javelin heads. It would take hours upon hours to just collect a few javelins which is most likely not worth it as you only get 20-40 max as a drop and it is not the most common drop either. Neither does accumulator re-collect them much like bolt racks. Smithing them only gives 5 heads per bar thus its pretty pointless to smith them as smithing dart tips out of your bars are the main focus on ironman accounts.
I would really love to use my ballista so as my suggestion follows:
-increase the amount of rune + dragon javelins heads as a drop to a respectable quantity
-add adamant and amethyst javelins or javelin heads to ironman ranging or legends shop for a respectable price. Not too high price as they are lost every shot. Or have them sold at a decent price meanwhile having the accumulator collect them like any other ammunition.
