Try this link in Spanish, he he, you´ll find some slings from differents countries:

http://perso.wanadoo.es/hondero/museo.htm

Hondero

English
I saw in a local antique mall, a box full of perfectly round stones, somewhat smaller than a tennis ball. The story behind it was a man was tillering on his property and these round rocks unearthed. The price for each stone was a little expensive. So I moved on. I know they had to have been slingstones used by the native americans. Maybe they were tools used for something else. Growing up in Kentucky, every spring when the fields were freshly plowed, I would walk the fields and gather all types of arrowheads. Some of my friends would find large spearheads. I haven't seen any slings yet, but plenty of stones. I think I'll go back to that mall and buy one of those stones!

I found a site that talked about a sling used by the Chumash indians.  Looks like they used clay balls for ammo (sometimes). 
http://www.abotech.com/Articles/Campbell02.htm