We took our slingshot out to the woods and used Lemonhead candy ... the evidence disappears over time (and it's cheaper).I buy bags of marbles as sling shot load at .99 cents store. Best deal ever.
Yeah, they aren't perfectly round ... but much better than the rocks I used to shoot as a kid (Lemonheads are fat free, too). Of course, I don't recommend storing them for long term use.