Sanders, I managed to precure a load of Walnut shell, which is what is recommended. If you are anywhere near Oxfordshire I could let you have some.
A mate has used cat litter (Diatomatious Earth) which is very successful and lighter in weight than mine, but don't get it wet.
I use plastic pellets in mine and they are fantastic. Half the weight of sand, does not matter if it gets wet, and mould according to the surface you place it on. Unfortunately, I do not know where you can buy them from. I have tons of them if anyone wants some but you will have to collect from Kent.
The most solid shooting bag I ever encountered belongs to Muir, off this site. It is filled with lead shot! you don't want to carry the bloody thing too far but it is fantastic to shoot off, rock solid. i forget how much it weighs, but it is a lot.