Potentially, you can use so-called "distilled" water from the grocery store that's intended for use in reconstituting baby formula. Wal-Mart's "green cap" water might be suitable, or it might not (I don't shop at Wal-Mart as a matter of principle).
This is what Wal-Mart says about the water you're referring to:
"We have multiple suppliers that manufacture our products and formulations may change from time to time based on customer feedback and other business needs."
To me, that's not a statement that inspires confidence. Far preferable, IMO, would be to find a decent LFS that will sell purified water to you, though you have to consider whether that LFS is properly maintaining their system.
And as others noted, far more preferable is to get your own RODI system, because it's much cheaper than purchased water in the long run when you consider the cost of the water, the gas you're going to use going to get it, and your back complaining about toting around 40 lbs at a time.