This free eye material and model can be used to create infinite eye variations, and can be scaled from "toon" to realistic. You can even make them glow using a slider.
I've included a "toon," "realisitc" and "glow" versions in this project. The sliders and inputs are all labeled and it should be pretty easy to use.
CC0 - so do anything you want with them. As long as it's legal.