Southdown is now being sold as "Yardright/Old Castle Tropical Play Sand" so if you're lucky, you can find it at your local Home Depot:
The Crab Street Journal: Products: YardRight Tropical Play Sand
A good way to test if your sand is aragonite based is to soak it in some white vinegar. If it fizzes, it's aragonite based. If it doesn't, don't use it.