There is no doctor pepper in BA... I've been looking for root-beer and other off-the-beaten-track food products here for almost 10 years and can assure you that it just doesn't exist.
The soft drinks available here (Coke, Sprite, Fanta, etc) aren't imported, but manufactured in Argentina. I think that selection is limited because there is just no demand for new products...people are happy with what they have and aren't so willing to try new things. Import restrictions don't help (though I can't for the life of me work out how those el paso Mexican products, Twinnings tea, Lindt chocolate get through and so much other stuff just doesn't).
A version of root-beer is available, however, in Barrio chino. I think it's from Taiwan and it's called something like Ho-Chin Sasparilla - its in a crimson/brownish coloured can. Its not an exact match (less sweet, slightly more bitter) but desperate times...