Near our new place in Palermo Soho there is a store by El Portugués. They sell bagged flours, seeds, dried fruits, spices, cereals, dried beans, etc. in bulk (min. 500/1000 g per bag). The prices are very competitive, especially if you can share your purchase with a friend or extended family. They also sell directly to the public, but their require a minimum purchase amount (last time I went it was around 200 ARS) and you have to register with them at the cashier.
The best part is that they publish their prices and offers online, so you already know what you are going to find at the store. Not all stores cater the same selection, tough -- it fluctuates.
Example of January 2018 offers:
ALL STORES
