Not sure why you want to read it : ) but there is a newsstand at Libertad and Juncal (Recoleta) that has all the main US papers, although I believe they are usually a few days late. I buy a lot of newspapers and have never seen it anywere else. While it certainly isn't the same, there is someone who advertises in the Herald that can deliver the Sunday New York Times on Sunday here in BA.
The newsstand in front of the Alvear Palace Hotel at Alvear and Ayacucho would probably be your best bet to find the WSJ. They also have an incredible selection of foreign magazines. I don´t believe that the news- stand at Libertad and Juncal (where I shop) carries it.