Crafting "fake potions"
For a few reasons, I'd go with holiday ornaments. These are available in clear plastic, rather than glass, which is less of a breakage worry. They're also cheaper and lighter than glass jars. And they're already nicely spherical!


an example from Michaels craft store


Ditch the metal collar and ring.
Fill with a colored epoxy resin.
Add a decorative cork.


The main advantage of this is that it wouldn't spill. Also, if it does crack from being dropped or struck, the epoxy would hold the pieces together, reducing the chance of injury or mess.

But if you want the internal liquid to slosh about and look very potion-like, that's also feasible:


Ditch the metal collar and ring.
Fill with colored liquid.
Add a decorative cork, and glue securely in place.


That shouldn't spill either, assuming it's well-glued.