A well-known passage is the bit in Xenophon's Anabasis, where the Greek mercenaries fight off Iranian slingers and archers (using lead bullets). Xenophon also mentions slingers in his Cyropaedia, and Iranian slingers are often mentioned in e.g. the story of Alexander's campaigns or the wars of his successors and of the Hellenistic kings.

What other evidence is there for slinging in Iran ?

-R. Ghirshman (Comptes Rendus de l'Academie des Inscriptions 1967) mentions lots of clay sling bullets (found in the house of "an artisan" in Bronze Age Susa (say 1900 BCE)

-J. M. Casal, in his dig at Mundigak (near Kandahar, in Afghanistan-- but still relevant here) mentions lots of clay bullets in bronze age layers (CRAI 1952)

-"clay ovoid bullets" are mentioned for the fourth millenium from Miri Qalat in the Makran (S. Baluchistan, beginning of the Gedrosian desert)--R. Besenval, CRAI 1994

I'd be curious to find later evidence, Iron Age or even later (e.g. travellers or contemporary accounts)