Have you looked in a libreria? There's a few along Cabildo that usually have a full stock of notebooks, writing pads, etc. You may be able to find some there. Otherwise in an art supply shop they sometimes have stuff like that (and even in some of the smaller ferreterias they occasionally stock paper supplies). In Las Canitas there's an art shop on Soldado de la Independencia at about Matienzo or so that has a lot of things. There's also Grabatto at Solar that may have some stuff.