How big or small? Try CR's Craft online
CR's Crafts
They are a doll and bear supplier, but search on "animal eyes" and you'll see what they have.
My guy is a metalsmith/sculptor and almost all of his work is sea life, so he has boxes of eyes down in the shop and that's where he gets them.
Looks like 4.5 mm is the smallest. Then they go up from there. He bought a pair for a 12 foot long barracuda he made. They were huge.