There are several ball designs that fit your needs (and are commonly used for GBC machines) - a selection :

* [LegoBall, Bionicle Zamor Sphere](https://www.bricklink.com/v2/catalog/catalogitem.page?P=54821), and their various multi-colored variants (eg. [Ball, Bionicle Zamor Sphere with Marbled Yellow Pattern](https://www.bricklink.com/v2/catalog/catalogitem.page?P=54821pb04)) (16.5 mm diamater)

[![enter image description here][1]][1]
[![enter image description here][2]][2]
* [Ball, Sports Soccer Plain](https://www.bricklink.com/v2/catalog/catalogitem.page?P=x45), and their various multi-colored variants (eg. [Ball, Sports Soccer with Standard Pattern](https://www.bricklink.com/v2/catalog/catalogitem.page?P=x45pb03)) (17.9 mm diameter)

[![enter image description here][3]][3]
[![enter image description here][4]][4]
* [Ball, Sports Basketball with Standard Black Lines Pattern](https://www.bricklink.com/v2/catalog/catalogitem.page?P=43702pb02) (14.2 mm diameter)

[![enter image description here][5]][5]


  [1]: https://i.sstatic.net/x4Rh9.png
  [2]: https://i.sstatic.net/EP2fK.png
  [3]: https://i.sstatic.net/Y3k7D.png
  [4]: https://i.sstatic.net/8eezk.png
  [5]: https://i.sstatic.net/Ff9nW.png