At one time, I was trying to find the material Star Sound Technologies uses for their steel fill, since it makes common sense that the material is not proprietary. I looked on the McMaster-Carr website and found several polishing/grinding grits, and I suspect this is the material Star Sound uses. However, I could not determine which grit size matched.
So, there's a detective project for someone interested.
In the meantime, I purchased white play sand as recommended by others above. As has been mentioned, be certain to dry the sand before filling your stand. I put mine in a 2"-3" layer in a wheelbarrow and let it dry in the sun.
