Well, jolted into action by ADAXL's awesome pdf find, I tracked down a few articles for y'all through my uni library. I'm not too sure about copyright or even generic access without passwords, so I thought it'd be a bit dodgy to post up here directly. Pm me if you're interested in having a peek, and I'll see what I can do.
Content-wise, they are:
*An archeological look at Greek sling bullets
*Sling stones of biblical times
*A prehistoric sling from Lovelock Cave, Nevada
*Roman army sling use and finds of lead shots
*Tissaphernes' lead 'bullets'
Yay, isn't this all exciting!