I was there on Saturday morning, and there is a surprisingly good selection (and let's face it you always need a back up pair as you will inevitably lose the good ones half way through your hols) in the World Duty Free at the back wall of the shop on you left as you come out of Zone R security.

You can also use this voucher at T1 World Duty Free Shops, which might save you some money.

HTML geeks will no doubt choke when they see the URL for that link......