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.
Example of January 2018 offers: