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Not sure if it is the best place to buy, but I have seen many different kinds of sands at Petco. I believe that they are good for saltwater, but check on the bag just to be sure (if you decide to purchase from there).
I bought the white Petco brand sand when I had my saltwater tank, and now have the black sand for it. ( Going back to freshwater ) It doesn't blow all over and very easy to wash and doesn't cloud the tank. And it doesn't affect water chemistry.