jsolomon,
Southdown is just aragonite sand. Southdown is actually the company name, and the sand is sold as tropical playsand. It is the least expensive way to start a DSB if you can locate it. Since sand is so expensive, we try to help each other out by letting them know about good deals, and what has worked for us.
Most of Southdown's (I guess you would call them gravel pits) are on the east cost, and it is fairly common there. The further west you go, the more expensive it is to ship, so not many Home Depots carry it that aren't near the east cost. I have had a hard time finding in western PA.