Pool filter sand.... you can buy it at your local pool & spa. Its pretty cheap as well I bought 100 lbs for $30. Maybe you can find it for cheaper where you are from. But pool filter sand looks realy nice IMO!
If it will cheer you up: look up how much 100lb of moon sand or any substrate sold at a pet store costs!
Another thing to mention as I had some difficulty with for the OP to keep in mind: you want one that is advertised as chemically free. My lowes, home depot, ace, and the first pool supply store I went to only carried chemically treated sand as it is much better for doing it's original job of filtering pools!