3zub - If you are looking for an inexpensive way to fill your stands with a dense filler you can purchase "Play Sand" from your favorite home improvement warehouse store, or nursery at about $4 per 50 lb bag. It is not quite as fine and dense as silica, but it is safe and cheap. It may be a bit more difficult to remove if the holes in your stands are smaller though. Silica sand is pretty expensive in comparison, but is definitely finer and more dense (your stands will weigh more filled with Silica than with Play Sand). Kitty Litter, IMO, would be the least desirable as it would leave a whole lot of airspace unfilled. Other, more expensive, alternatives are lead shot which can be purchased from some sporting supply stores, or any gun shop that handles reloading supplies. I believe Star Sound markets a lead powder, which would likely be the heaviest and most expensive of all your alternatives. If you handle lead just make sure to wash your hands thoroughly afterwards. Same goes for Silica or if you decide to stuff your stands with cats. If you use Silica try to fill the stands outdoors and use a mask to filter out the dust from the air you are breathing. Even a foam painters mask would be an improvement over none. If you are using cats I'd suggest long gauntlet gloves and some of those foam earplugs.
Good luck!
Marco