I wouldn't trust the water dispenser at any store to be RO. most of them are just carbon filtered for chlorine removal and to make the water taste better. At best they produce marginal quality water. Certainly not good enough for your reef tank. Getting your own RO/DI system is by far the best approach and will give you some peace of mind knowing your putting the best quality water in your tank. They are a pretty reasonable investment IMO. The problem with well water is that it can be high in Nitrates. I deal allot with this in my area. Most people have no idea they have a problem because Nitrates are not detectable by taste or odor. How long has it been since your well water was tested? Good luck
Ya its not feasible for me either at the moment. In 6 to 9 months possibly. The machine I use has sticker on it from the guy who cleans it everyday. It says it is sediment filtered, carbon filtered, ro filtered and uv filtered at the end. I did a tds on it and it read 5...not 0 but way better then the tap and cheaper then distilled.