Good distilled water is esentially the same as good RO/DI, just a different treatment method. Both should be at or near 0 TDS. I say good distilled though meaning a reliable source same as you would want with RO/DI, the treated water is only as good as the equipment that produces it and the maintenance done of the equipment to keep it in shape.
Most users report that the purple cap one gallon jugs of distilled water from WalMart are pretty consistent. Some will say look out for copper but copper coils and stills have not been used in decades in large scale distillation systems.
Either way, if you plan to stay in the hobby and cannot afford a RO/DI system at this time at least spend $25-$30 on a good handheld TDS meter such as the HM Digital TDS-3, TDS-4TM or AP-1. all are ATC temperature compensated and very versatile. Meters you find for less are not usually ATC compensated and are not as accurate. If you have a very large reef tank and are keeping up with water changes it does not take long at all to pay for a $125-$140 reef quality RO/DI system compared to distilled or store bought waters at .89 to $1.00 a gallon or more. Besides that if you drink bottled water you can really pay for a RO/DI in a hurry by refilling plastic water bottles and jugs with the RO only portion of RO/DI so it serves a dual purpose or you can add a pressurized drinking water kit and a check valve to any reef RO/DI system for less than $65 or so and have the best of both worlds.
