Yes I have seen them for sale in Los Cris.

You will need a"prescription" only in the sense that the optician needs to know what strenght of lenses to give you. You should be able to obtain a copy of your current prescription from your UK optician assumming you have had a recent eye test.

Mine is -6.5 in each eye and I would just take an empty box with me if I needed replacements.

Thanks for info Peter we will be staying at parque santiago 3 so will have a look around there.
Petra