Antártida Argentina 1355. It's a short walk from the Retiro station (but it will help if you look at a map first).
Ask for the prorrogas de permanencia (you'll really impress them if you prounounce it correctly).
Be advised that if you get the extension there, you will NOT be able to go to Colinia and get another prior to the expiration of the extension.
If you plan to leave Argentina within the 90 days granted by the prorroga it's probably the best course of action, otherwise you may benefit from reading some of the other threads on the subject.