Getting some bits for my car the other week, I asked if I could have a few wheel nuts from my local scrappy. Hard, dense, free, pretty much useless to anyone else...

Golf balls can be had cheap.

How about casting copper, rather than lead?

Rather than moulds, why not dropping 2oz amounts of molten lead from your garage roof into a 50 gallon oil drum filled with water...sort of like how they made bird shot up to .38 a few centuries back. Yes, they did find that dropping the lead from higher heights did improve the spheres produced, and produced towers to drop it from, visited one in Virginia.

I still think treating the world as your ammo dump makes a sling a wonderful toy.

Justin

Have you ever tried soapstone? It works great for molds, and will last forever if taken care of right.
  Personally, I would put a smiley face on one side of the bullet, and on the other side, " Pleased to meet you."
   Have you ever thought of using either pewter or Brittania metal for the slugs? They are easier to get, and you don't run into the problem of antimony poisoning.

nightweave, do you mean a track ball mouse or a normal mouse?  a normal mouse ball is  a little grey rubber thing.  track ball balls are big (there is a track ball company that makes trackballs that you can replace with pool balls) dense plastic things.  i have thought many times
"what good ammo this smooth dense hard sphere would make for my sling."