Pool Filter Sand at Lowes outside with the pool supplies. $5 for 50#.
Some have had luck with the cheap scentless Kitty Litter from Walmart as a clay substrate. Proceed at your own risk with that stuff though.
x 2 for the Pool Filter Sand. It's cheap and way easier to clean than the play sand. Play sand is a nice tan color and looks great, the Pool filter sand is white and IMO looks awesome against the colors in a planted tank.
We chose play sand for our 125g... it was a pita to wash... it was dirty dirty dirty. We got pool sand for the 20g and it washed up much easier.