Pool filter sand hands down. just go to a pool supply store and ask for a bag or two depending how big your tank is. cheap to, last time i checked it was 10 dollars for a 50lb bag. color ranges from white, brown, tan or a mix of the three. the grain is large and heavy enough to be cleaned with a "vacuum". just make sure you rinse it before adding it to the tank.
oh btw. make sure you make this project a good week or two. if you take all your gravel out at once your tank will lose some bb colonies and go through a mini cycle.