Yes, another vote for Ostramar. Jumbo supermarket has decent offerings from time to time, but this depends on the season. I have another 2 shops in mind but they are small non-chain stores and I dont have the exact addresses, I just turn on the auto-pilot and go there.... one is on Cabello close to Nemo (the restaurant), the other is in Martinez so its probably of less use to you.

One place that has excellent fish is the Mercado/Feria Municipal de Belgrano..... overall one of the best food markets in Argentina, with the best of what the local produce & meat markets can offer and also plenty of imported things flown in. Pricey... but well worth it.