that depends on you. water from you LFS, if it already has salt in it, i believe this is what you are tlakign about, would be more expensive. I know that some that do sell tank water. This is good in the sence that there will be bacteria already in there to help the cycle. The only problem is it would only be good if they have good water quality. You might want to try a jug and sample it. If you buy it form the grocery store you will know that all the levels will be good to go. But if you are cycling your tank and there is nothing in there, AND you have the extra money, then your fish-store water would be good since you will have benifial bactera, and amonia and such to cycle your tank. But once it is sycled and if the water quality is bad, use store water.
if you do buy it form the store, then your alinity level should be between 1.022-1.024. even higher is good, but much lower is fatal to some inverts