This link has all the Clothes that are available in Pokémon X and Y.
http://www.serebii.net/xy/customisation.shtml
I thought I would post this, instead of telling you good Clothes locations since it would be my opinion and also because we might have completely different tastes.
Each Boutique is different and has different Clothes you can buy but in my opinion I think Lumious City, Anistar City and Cyllage City are rather good places for buying Clothes.
Hope I helped :)
