Might be a tough nut to crack. I just checked (on the internet, so latest information possible) on three wine stores listed in a guide I have, (Maria Cabella, Lavinia and another) and none of them are open Sundays. Good luck!
My wine shopping and I have never had a problem. Just a perpetual race between the bankruptcy court and Hell.--Rogov
Sundays are tough in Spain as Jenise mentioned. A fair bet, though, would be the wine department of Corte Inglés on Preciados Callao (see: http://www.elcorteingles.es/promociones/preciadoscallao_0108/preciadoscallao.asp). Best to call first to make sure though, but there is a good chance with this higher-end dept. store chain. I've been to several of their wine departments in Barcelona, and they have a fair-pretty good selection of wines.
I guess I will have to settle for El Corte Inglais or the duty free, although I just realized I will have a few hours to kill on Monday, since my work should be completed by noon and the next flight is at 7 PM.
Positive Discrimination For White Wines! http://2GrandCru.blogspot.com