Thw best sand for ballast dampening is?

I am buying a sandbox amp stand for preamp. My only experience is using steel coated lead shot. I was given the shot for VR2s which have a sealed off compartment for that purpose.
I have heard that there are very dense shot like materials that are better than either fIne clean sand or lead shot.
Anyone know what to use and where to find it. I am curious about this new material I think it is a black color metal alloy.