That's a good question - you can ask for RO/DI water in an aquarium store. They know what that is. Some only sell RO water.
In some grocery stores there are units that produce RO/DI water - it may be called "purified water" or some such thing, but the process will be described and it should refer to "reverse osmosis" (RO) and "deionization" (DI) as the purification processes. That's what you need to look for. You may have to buy a water container there, or bring your own.
Some water sellers that deliver also have this water, and our members have found it in 5 gallon jugs in many types of stores.