The Guia T is the most handy guide for getting around Buenos Aires. They may be found at many of the newspaper stands around the city. Another brand is called the "Guia Filcar", which is also helpful. This now comes in a smaller version like the Guia T.
Hint: If you can't find a Guia T on the street, try riding the Subte or trains; you are bound to find someone selling them up and down the aisles there!