Go to a feed store and buy a bag of mineral for cattle or pigs. It is cheap and just as heavy.
Would lead shot not work well?
Perhaps 7 1/2 or smaller?
Several good leads here, thanks for all the replies.
Your looking for Alumina Zirconia called stone blast from Norton abrasive it's used for blasting Granite Headstones.It's $150.00+ for 50lb box= .40cubic ft. 8"x9"x10"box probably at least 2-3X as heavy as any chit that comes in a BAG! Check your local Memorial monument manufactures or you could buy direct fromerusha Supply Stansted Quebec (819)876-2722 .Used product will be extremely fine (repeated use) until it drops through a screen, so you probably want new product.
Actually Garnet has a higher density than Alumina Zirconia (147 versus 130 lbs/ft3) and is generally easier and cheaper to get from blasting supply stores.