I would never buy water from anywhere, including my LFS, unless they test the conductivity or TDS in my presence or allow me to bring my TDS meter into the store. I have seen too many people get burned with bad water.
There is only one LFS in the entire Phoenix metrao area I would trust and even they have problems. Surprisingly though what I have found is some of the Water & Ice type stores have very good water quality for RO only water, by good I mean getting it from my tap water reading of 800 down to less than 10 TDS. Still not as good as RO/DI but pretty darn good.
If you must purchase water find a trusted source who will test it if you ask. I found the closest Water & Ice store was very interested when I explained it was for saltwater aquariums and fell all over themselves being helpful. They were proud of their store and equipment and it shows. On the other hand the closest LFS, in the same strip mall as the water store in fact, has horrible quality water and its represented as RO/DI.