Ceviche, can you invite us all over for dinner sometime? Sounds like you are cooking up a storm! I have had luck with getting black rice at one of the places on Arribenos... So sorry I have no idea what it's called... It's one of the smaller ones between Juramento and Mendoza that has all of the canned goods and teas in one side and then on the other side is bulk goods and frozen foods. But they don't have it all the time. it is usually in bulk goods and sometimes prebagged. Otherwise there are two supermarkets in the following block, btwn Mendoza and Olazabal that are on the same side of the street (the side closer to Libertador) that have relatively large bulk sections of nuts flours rice, and they may have it as well.
The whole wheat flour here is kind of different, it seems more raw than back in USA. It almost seems as if they throw the whole germ in there and don't break it down even a bit (kind of like with the rolled oats, the gruesa variety is less processed and your apt to find the casings of the oats in them, I always pick them out) You may have luck with finding a different variety of whole wheat either in Chintown, at the Casa de Polti on Juramento and about Ciudad de la Paz, in Natural Deli, at Hausbrot itself (the do sell oats etc) or a casa de reposteria if you are lucky. there is a baked goods supplier on Corrientes I believe it is, have a look on the forum, I cannot for the life of me remember what it is called, sorry!