This is a paid asset, but now you can download Eye Advanced Free.
Detail this asset from Unity Store: Original Link
Eye Advanced v1.1 (Latest version)
“If possible, please buy the package to support the developer”
EyeAdvanced is an Eye shader and prefab system that brings brings realistic eye rendering effects to Unity, including advanced specular, parallax warp, dilation, and physically-based shading algorithms.
Automatically change geometry and shader complexity as distance from eyes increases, or alternatively select one of three set LOD complexity options.
You have control over many aspects of eye rendering, such as eye color, pupil dilation, iris size and iris tint color. Includes 15 preset eye texture options, or use them as a template to create your own.
Scriptable Render Pipeline Support
Standard Renderer (Unity 5.0+)
URP Universal Render Pipeline (Unity 2019.3.0+)
HDRP High Definition Render Pipeline (Unity 2019.3.0+)
Unity 2020.x / 2019.x / 2018.x / 2017.x / 5.x
Unity 5.3.4 or above is recommended.