Just distilled (or demineralised) water is fine - no need for additives, which aren't allowed for racing anyway (St George is strictly 'water ONLY').

You can get demineralised water from Woolies or Coles (ironing section, for use in household irons so they don't block up with little mineral grains after steaming). Or bigger bottles are available at Bunnings etc, or Supercheap etc.