The sunglasses sold online are much cheaper than that sold in physical stores. Why not buy a pair of good sunglasses online? I have found two online websites for you. If you have any interest, you can have a look. Good luck! http://www.sunglasses-shop.co.uk/inspire-and-guide/travel-guide/marseilles.aspx http://www.etniabarcelona.com
Since I don't know where are you living, I don't know whether this store near your home. If you have to spend an extra traveling expense, that doesn't worth it. So I suggest you to have a look at your local eyeglasses stores. Compare their prices before buying them. This will save your money. Hope this helped!
I have never been to Barcelona, so I don't know where to buy the cheap sunglasses. Hey! Are there many famous football players? I would buy a pair of sunglasses and watch them play football on the playground. I think it is easy to find a pair of sunglasses. You can ask the passersby on the street. I often do that when I can't find the way or I want to buy something. Some of them are really very helpful.