If you want pure water, use steam distilled. The pH is a little low, but it has zero buffering capacity so the pH is irrelevant. RO water can be fine, but it depends on how it is done and the amount of impurities that are removed. RODI water is even better since it is reverse osmosis deionized. Nearly nothing remains but the water.
If you want cheap off the shelf...buy steam distilled from the local supermarket.