How to re-lens some Rayban Caravans?

I have some older Rayban Caravans I'd like to add prescription lenses.

Can anyone tell me the best way to do this? I tried my local optometry shop and they only offered Leica glass instead of the authentic Rayban ones. I'm being vain, I want the logo.

What's the best way to get authentic Rayban prescription lenses?

I can't seem to find a way on their website to order just a set of lenses, they want to steer me into a whole new set of sunglasses.