I'm guessing the only place where you might find them is eBay as outside of Rolex's own service center, I do not know a place/shop that openly sells them.
You wouldn't even find them in a Rolex dealership.
Also bear in mind that, depending on the model, the cyclops might be further away from the center than on other models. Make sure you have a gen-spec dial if you install a genuine crystal, or your cyclops might end up magnifying god knows what.