[Ar-Cunin]

And if you have the bonus merchants, you can buy a +5 sling that doesn't require bullets - a very good weapon.

It is still useful to have one mage (if you have more that one in your party) that specializes in darts, so that you can take advantage of the darts that you find - after all darts have 3 attacks/round, where as slings only have 1 attack /round.

Finally daggers: In the underdark you can buy a +3 throwing knife (returning) - with 2 attack/round it is much better than a sling.

But in the beginning the will need the sling the most.