I got my sand (play sand that is safe for children) from bunnings. It was $9.35 for a bag. To fill a clam shell thing i only used 2 bags.
We used that too, it so fine and feels GREAT.. you can use a tarp, OR get the measurements and if you know someone handy get them to make a cover that stetches OVER the sandpit, you can use netting but it wont stop animals peeing in it.