Something I haven't looked for here, but my first thought would be one of the larger, Dietetica Stores...if they don't carry it they should be able to point you in the right direction...since these are mini health food stores they just might carry it.
Awesome! I found a great health food store today on J.L. Borges, just a ways in from Santa Fe street. Everything from aloe vera to stevia to whole grains and spices, it was all there for fantastic prices (well, not the aloe vera)! Just wanted to let people know in case they're into that kind of stuff : )