I think the OP is looking for heavy sand (zircon or chromite) to fill a rear bag, something that M40_A1 eschews when he shoots F-class LR competitions.
I would certainly not call it a squeeze bag, more like a rear rest. When I bought my Protektor 5-7 years ago, I bought the 20lbs heavy sand from Sinclair at the same time. I still have 5 lbs of it left over somewhere. Sinclair has been out of stock for it for a long time now as I have been considering a larger rear bag. A cursory search on the Internet shows that it costs about $150-$500 a ton, depending on quantity, and there are minimum orders for several tons. XTR has a good idea looking for a place locally that uses this stuff in quantity and see if you can buy 20-30 pounds from them. It's used in foundries, for example.