So your looking for a good quality ash ballast. I would recommend finding a small iron works and getting hold of some of their waste casting sand, they'll give it to you free of charge, just because it costs them to get rid of it. It makes a good ash ballast as I've used it before myself. Just remember to take a heavy duty sack or bucket with you, Your going to have to put it through a sieve as some of the sand sticks together during the casting process, so get more than you think you'll need.
You could also ask about if there's an artist in the area that does cast bronzes they'll have some too.
