Have you tried the Dieteticas? There are loads of them, pretty much one on every street in my neighbourhood. Some better than others of course. They have plenty of gluten-free products, although I have never looked specifically for rice milk so can't guarantee that they have it.
Here's a few addresses from a website I just googled:
[*]Dietética Casablanca - Malabia 351
[*]Dietética Casa Polti - Juramento 2499
[*]Dietética Juramento - Juramento 1598
[*]Dietética Arco Iris - Av. Cabildo 3230
[*]Dietética Compostella- Migueletes y Lacroze
[*]Dietética Don Gaspar - Monroe 2789
[*]Dietética Alma Zen - Malabia 484
[*]Dietética y Bar El Artesano - Jean Jaures 1197
http://www.dieteticaviamonte.com.ar
Also try
http://www.grandiet.com/ -- there's one near me that's very good, again not sure about Rice milk specifically but you could give them a ring -- they don't post all the locations on the site it seems but they should be able to help you out.