Just a really simple item, to complement the magical sling bullets I posted way back, this insure you have everyday ammunition. Personally, I feel the sling gets a short stick in D&D/Pathfinder and I may post my variant rules for them in my campaign. But, in any case, here is:
Pouch of Perfect Stones
These tough cloth belt pouches are reinforced with leather and often patched.
Sea of Stars RPG Design Journal 