If you buy from a supermarket, get distilled water. Bottled RO water is normally for drinking and has minerals added in for taste.
If you buy from a LFS have them test the TDS in your presence every time so you know what you are getting, LFS are notorious for poor maintenance of their RO systems since it is not a money maker for them in most cases. There are always excepions but this seems to be the case most of the time.
Having your own RO/DI is by far the best choice, that way you alone are in control of the water quality.