Some of the best things that you can get are almost always free when it comes to software !
This is a video by David Connely and he has added an download link, so you can
download those procedural created eyes and create different types of 3D eyes
in Marmoset Designer.
I can't be greatful enough for this and I would like to share it on this platform too.
Here is the link to download his substance file and eye meshes :
http://www.davidconnely.co.uk/Free_Substance_Eyes.html
Follow the video to see how to setup everything !