Shop Floor Tile at Lowes.com
This is the section I bought mine from but can't remember which one it was. See..they do have very natural ones and they will sell them seperately and trim them a bit so you can get an exact fit. And other similar stores will do and offer the same. Measure the inside of the tank and then go from there. You will love the results!
You seem very determined about the sand in his viv... it's your life and your beardie so I'm not gonna try to tell you anymore what to do. If you do ut a dig bin in there. Toss the whole contents if he poops in it (well that's also up to you) but it is only that I'm am offering you the best advice based on his health. I'm not your sister in law so donn't get upset with me. I know you love you beardie. Just take what you want to from the advice I am offering you and enjoy!