I make my own eyes from white plastic beads, so they are spheres. If I got those I would probably cut and sand the backs of them so they are perfectly round. That way they can rotate in the sockets.
For build-up heads, I use a bit of epoxy putty for the socket - I press the eyeballs into them to form the socket.I rotate the bead as it sets, so it doesn't stick. Then I paint liquid latex eyelids over the beads. When the latex is dry, I peel the latex away, and pull the bead out. Then I dust the inside with talc, and pop in the finished eyeball.
For heads cast in a mould, I have an eye bead in the clay sculpture for the eye. When I make the plaster mould, it moulds the curve of the exposed part of the eyeball. Then I remove the clay and the beads, and get the mould ready to cast in silicone or foam latex. I usually drill a small hole in the mould, where the pupil will be, so I can put a bead on that, fitted to the curve of the mould, so that when I cast the head, there will be a hollow socket formed where the bead is. I also put an armature inside the head, but it doesn't usually have eye sockets in it. Both methods can be seen at my Youtube channel.
The build-up head: https://www.youtube.com/watch?v=4uG-ZIOXZ7w&t=32s
The cast silicone head: https://www.youtube.com/watch?v=5fWdZnQRzB0
StopMotionAnimation.com has been hit hard by spammers lately. They have been creating several fake accounts every day, contacting and harassing members and posting suspicious content. If you see anything suspicious, don't click on any links, instead, send me their User Name. I will suspend them.