Where can I find foil/mylar balloons that are smaller than 18″?
Probably hard to do, at least if you expect it to float in air. Those balloons need to have a sufficient amount of helium in them to float the mylar. If they’re too small, there’s not enough helium in them to float the mylar. (Surface area is a function of a square and volume is a function of a cube).
I’ve never seen them smaller than about that size, except the ones attached to the ends of sticks and put in flower arrangements, which aren’t intended to float.