Who sells lead shot pellets to fill an equip.stand

Clay based Kitty Litter is widely used...........

A good option may be the stuff they use in those outdoor ashtrays in public places. It is denser than play sand.
However, is is around $15.00 per box and may get pricey
depending on how much you need.

That lead is toxic is a valid point. If you 'loose' some into your carpet, or where-ever, it is something that can harm you, your pets and your children. When you dispose of it, it is gonna be hard to get rid of (toxic, remember)
So I agree with all the alternatives as being just as good. (Especially if you ever intend to move them! Weighing 250lbs each is harder to move a few feet or even a few inches! than 60/100lbs of sand or whatever.

Even micro-scopic amounts of lead dust might cause permanent neurological damage to developing brains many years later. Don't use lead!

Lead even in small amounts may certainly be toxic.
If you do not want to use sand, Tungsten shot or spheres
are a good alternative. Tungsten is unfortunately
expensive. Do a search online for ordering
info.