Good luck finding soy milk.
Although Argentina is one of the top 3 or 4 producers of soy in the world, the concept of "Soy Milk" hasn't reached her yet.
There are some "Natural Foods" type stores on Cordoba between Parana & Montevideo on the north side of the street. (The "RIGHT" side as you are driving with traffic.)
One is a restaurant at street level with an escalator that goes downstairs to a little market. There's another store one or two doors down.
Will update as I remember things.
It is near the -- a -- market. I've forgotten exactly and am not in Bs.As. at the moment (just back, can't even find my "Guia 'T'") but think it's on the western side of Juramento, about two blocks south of Cabildo: very near estación Juramento of the subte.
Was just at Verde Brote today. Great place. Juramento 2679. Keep meaning to eat lunch there-- lunch looks healthy AND affordable. a few doors away is another great one-- smaller, on the corner -- spices, beans, grains, nuts, chocolate, etc. It has more feeling, but less selection. Casa Polti: Juramento 2499 While you're in the neighborhood food shopping, you should check out Casa China supermarket. They have many of the same Chinese products for less money. Verde Brote seems less geared towards the Chinese shoppers, though probably Chinese owned.