Look for "dieteticas." They are all over the city, but not all have the same quantity of products. All will have cashews. I think I saw spirulina in a few places. In general, your best bet is in Barrio chino (tofu, miso, and most other things of Asian origen).
If you are located closer to the center, the largest dietetica I have seen is Dietetica Viamonte at Viamonte 859.