If you find yourself in Recoleta, there are two places that might have it:One is a little shop at Libertad 1157 (between Santa Fe and Arenales) called "Diki."The other is a shop in a galeria just off Montevideo (between Santa Fe and Marceto T.) called Le Gout. The exact address is Montevideo 1079, #27.
I've been to both, and would be guardedly optimistic about your chances of finding what you're looking for. In general, a good place to check for specialty stores is Dan Perlman's blog, Casa Saltshaker. He has this one particular page on which he lists a wealth of food-related resources...http://www.saltshaker.net/buenos-aires-food-drinkThere are several other baking supply type places on that page. Good luck.