Black Mexican beach pebbles look pretty cool and they lay decently flat on one another. Just make sure you wash them off real good. You can use a typical strainer from your kitchen filled with them under hot water coming from your tub. Keep rinsing until all the tub water runs clear. I'm pretty sure they hold water pretty well too. Depending on the season you can probably find a 15 lb bag at Walmart, and I believe Lowes and Home Depot carry them regularly. They are pretty heavy though so make sure you take that into consideration when setting them up; strength of bottom of cage, collapsing on animals, etc
