Someone in the industry who I respect a great deal, but don't have permission to quote so I won't use his name, thinks very highly of Star Sound Technologies' Micro-Bearing conductive steel. Someone other than the folks at Star Sound, I should add!
http://www.starsoundtechnologies.com/micro.html/
I speculate that a poor man's alternative might be uncoated small-grain aquarium gravel (some aquarium gravels have a plastic coating, which I think would go counter to the philosophy of the Star Sound steel fill).
Duke
dealer/manufacturer