I've brought back many pounds of rock from Florida. All of south florida was at one time a massive coral reef, You can pretty much dig down a little anywhere and hit aragonite.
I've found so much coral rock laying on the beach. I have not paid a penny for any of the rock in my two 20G longs. I will say it is hard to find big pieces of rock on the beach. Also, it almost goes without saying: do not rip live coral rock off the reefs. The best rock I've seen was in the keys and southwest Florida. They keys are made entirley of dead coral skeletons and the quarries there have excellent dry rock.