I have been to the "librerias turisticas" en Paraguay/Pueyrredon, a nice little shop full of guide books of Argentina and the rest of the world, and also maps (behind the counter). I can really recommend this shop for anyone going to travel in or outside Argentina.
They only had 1 of the provincial maps I needed, and no trekking maps.
So I went to the Automovil Club, (where you don't have to be a member, although I think you get a reduction if you are). Here they had all I needed, including trekking maps.
You have to know what you need though, as you can not "look around and see what they have" but you must explain what you need. Then the man shows the map and you can decide to buy it or not.
There was no one in front of me, but there is this ticket machine to get a number so I guess it can become quite busy.