One of the more common places is reef tank suppliers.
HERE is some. However, a MUCH cheaper alternative (if you're in the right area of the states) is the SouthDown or Yardright brand playsands. Check your local Home Depot for it. It's a playsand for sandboxes, but it's actually collected in tropical waters, so it's actually an aragonite sand. Check this out:
http://www.crabstreetjournal.com/produc ... ysand.html
