Ok, well sort of being able to find a ball on its own, and i did a search and could find nothing, or being able to make one yourself, the otehr option could be this set of figures from master Box.
https://www.hannants.co.uk/product/MB35149
I know its a lot if you just want the ball, but there looks to be a child with a dog there, so you might be able to use that figure.
I am a Norfolk man and i glory in being so
On the bench: Airfix 1/72nd Harrier GR.3/Fujimi 1/72nd Ju 87D-3