Mason's sand is perfect. You can get it from any sand and gravel dealer/quarry. Color, of course, doesn't matter, but it comes in black, red, yellow and white. It's smooth, clean and even, and doesn't cost too much. You buy it by the yard, which, by definition, is a cube 1 yard on each side. But measures are never that exact. Still, one cubic yard will cover 9 square yards (9'x9') to a depth of 4".