Silica sand that is used for flower arrangements is supposed to be good.
I buy the muslin bags filled with sand at a LQS..but I just read somewhere, when you use sand in some kind of fabric if you wrap it with interfacing it wont leak out. I usually wrap leftover batting scraps around the muslin sand bags to plump them a little.
Also, dried rice or beans would work...but I would roast them in the oven a while because I would worry about bugs getting in them....retired nurse fanatic about germs/bugs lol.
Margie