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
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