Cochinito. I suggest you go to one of the retail wine outlets, two good options are:

Winery - they are all over the place
Frappe - with 4 locations in Buenos Aires

Ask your concierge for details. By all means, please avoid Ligier - they are thiefs but unfortunately grease most hotels so that the concierges send the poor tourists there... they are really bad people and should be shunned. Please take this from a veteran of the Argentine wine scene... you dont want to buy there.

As for wines, whats your budget per bottle? I think that for US$ 15 a bottle you can get a heck of a bang for your buck. In this price range, I suggest:

Cabernets
1) Carmelo Patti, Cabernet Sauvignon
2) Trapiche, Fond de Cave
3) Escorihuela Gascon, varietal line Cabernet Sauvignon

Merlots

1) Escorihuela Gascon, varietal line Merlot

Malbec (Argentina's signature grape)

1) Domingo Molina, Malbec
2) Trapiche, Malbec Reserva Especial

If you want to go upmarket... there are many. Just check out the wines from the following wineries:

Catena Zapata
Finca la Anita
Terrazas de los Andes
etc.

Hope this helps....