Any clay kitty litter will do fine. I just got a pair of Skylan 4 post stands for my Monitor 30's, filled them about 2/3 full, 5 pounds per column, with unscented store brand kitty litter. I've been using kitty litter in metal stands for years. It was recommended to me by a guy who has been a high end dealer for over 30 years. At first, I was surprised that Noel recommended it, since Skylan stands don't ring the way metal stands do and it seemed to me that the only reason to fill the columns is to add weight and mass. But the more I thought about it, the more it made sense.
Anyway, the Skylan stsnds are terrific. Big improvement in bass definition, clarity, soundstaging, depth and imaging. It's like the speakers are suspended in air, but are absolutely stable. Terrific jump factor (which I noticed when my dog jumped).