For the kid scenario: I'd ask the parents where they found the ball and if they saw any more. If they are nice enough to answer those questions I'd add in a "boy I wish there were more because my robotics team could sure use them" If they ask "Why?" I'd tell them and introduce the kid to FIRST at the same time. Then I'd offer to make the kid a ball in his / her favorite colors in exchange for the real ball. But I wouldn't push too hard, that would just be creepy.
Lots of balls in a store... at this point if I was lucky enough to still find balls, and my team needed them (which we don't 'cause we are already making them by the trailerful) I'd buy them all up and then send an e-mail to all the local teams to tell them we found some more balls and see if they wanted some of them. Then we would share!
If I ran into another team in the store, I'd share on the spot! Even if I only found three, I'd definitely give up one of them!