I was finally able to visit all three places on my search for mochi and here is what I found:
1. Tienda Ohno - small shop in the Belgrano Market one block from the Juramento subte stop line D. Unfortunately I arrived at closing time, but if you've not been the Market is interesting with large indoor food court, food shops, and a big beer garden. Located in the Belgrano R neighborhood along Juramento and a nice area to visit.
https://www.facebook.com/MercadodeBelgrano/
2. Casa China - again, timing not so good. Went on an early Sunday afternoon Feb 2nd, the last day of the Chinese New Year celebrations in BsAs. Filled with people shoulder-to-shoulder, no mochi, but great place for hard-to-find Asian items. They did have a big variety of pre-made sushi rolls, about 15 cent. long for around $200 pesos.
https://www.facebook.com/CasaChina/
3. Nueva Casa Japonesa - mochi success here (both fresh and in an ice cream version) and highly recommended for Japanese items. Not nearly as busy as Casa China, and they have a very busy casual restaurant too. Just a few blocks from the Humberto Primero subte stop on line H.
https://casajaponesa.com.ar
If you've never tried one, mochi is sort of the Japanese version of an alfajor without the dulce de leche.
Archi and Bajo, thanks again for your recommendations!