I don't think any grocery store sells straight RO/DI water (where you have 100% control of what gets put back into it). That's because zero TDS water tastes like crap and wouldn't sell very well.
What you will find is filtered RO water with a bit of TDS at "u-serve" type filling stations (in Superstore, for example). This is fine for our tanks but you're reliant on someone else to make sure the filters and RO membrane are adequately maintained.
You can pick up distilled water at the pharmacy department, which should be zero TDS (or about as close as you can get), but it's a lot more expensive.
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