You would want calcium carbonate based rock. Usually this means that at one time it was in the ocean and has the calcium carbonate base that is ideal for out aquariums, just like Fiji rock. I purchased mine online, even shipped it was just over $1 per pound. Seems that when they expand harbors like in Hawaii they just dumped the rocks in piles, someone figured out that they could sell it and they do.
I add some from time to time to my tank and have no problem with the water chemistry because of it.
Thomas