If you come out of the main road (dual carraigeway) to the roundabout, instead of turning right to Consum / Mexico etc, turn left and head towards Villerocos. There is a petrol station a short way up (by Palamores R'bout ?) - This sells Magazines / food / milk + drinks etc - very similiar to any large petrol station over here.
Sorry - can't remember the exact opening times, but certainly Sundays and late - although I don't think 24hr ?
Worth a look? - hope it helps.