Stocking as in balloons, impromptu as in "these two airheads completely forgot to actually bring enough balloons to put up for a 'free balloons' stand, so they're making do with whoever comes asking for them'.
Technically speaking they'd be able to just poof a bunch of them on a whim if they really wanted to, but what's the fun in it when they can just pretend the "free balloons" means more than just getting one?

This was meant to be a quick silly thing, but I just felt it'd look nicer with some color and whatnot. It's still rough on the edges, but eh, oh well.