Over the years I've used all kinds of things for eye forms (depending on the scale of the character) everything from ping pong balls to marbles.
One thing I would suggest is google searching "acrylic spheres" and you should find a lot of plastics manufacturers that offer clear spheres in various sizes. The work great as is, or can be easily painted to create quick eye forms.
If you want to go a bit more realistic (which can help with likenesses, emotion/expression, character, etc) you can use more realistic eyes. Taxidermy supply companies offer a wide variety that can work for different creatures. https://www.vandykestaxidermy.com/ is a popular source for those.
Answers
One thing I would suggest is google searching "acrylic spheres" and you should find a lot of plastics manufacturers that offer clear spheres in various sizes. The work great as is, or can be easily painted to create quick eye forms.
If you want to go a bit more realistic (which can help with likenesses, emotion/expression, character, etc) you can use more realistic eyes. Taxidermy supply companies offer a wide variety that can work for different creatures. https://www.vandykestaxidermy.com/ is a popular source for those.
For realistic human eyes I would suggest https://fourthsealstudios.com/
Hope that helps!
/Chris