I use Qrock from our local brick/masonry supply company. It comes in four sizes 0 through 3. Zero being VERY fine to #3 being coarse. I usually use #1 or 2. I have also used regular masonry sand that I bought from Home Cheapo, but you have to screen it before use to get the big stuff out. I use regular window screen.
THIS IS VERY IMPORTANT - Use a respirator/mask if you use anything with silicone in it!! That includes what I listed above. This is to prevent you from developing silicosis.
Some here use black beauty that you can purchase from your local Tractor Supply. I would not buy it on line. The prices are outrageous and you have to pay for shipping. Do you have a local quarry? You need to find a local source, because you will go through a boat load of it
Hope this helps