Stones/pebbles, standard marbles I'd think (they don't have the same build as sling bullets), anything picked up off the ground. Look through the standard, core equipment list and find anything small and light enough to carry in a sack. Caltrops? Torches? All fair game, I imagine!
Also, yes, I'm ok with a quick jester. That's the point, no? You'd also likely be the one thing in the world capable of giving me character a chuckle.