There are loads of new specialty coffee shops roasting their own beans. They're more expensive, but the quality is better than their grocery store counterparts (in my humble opinion, of course). You can also talk to the folks at the shop, tell them what you're after and get a bit of advice.
Rita, Cuervo, Negro Cafe, Full City Coffee House, Delirante, Blanca, pretty much any 'cafe especialidad' popping up has their own brand or uses someone local. You can pop in and ask them. You can get them full bean or they'll happily grind them for your French press. I use a volturno every morning and I try new bags of coffee each week or two from rotating shops. My favorites so far are: Fuego Tostadores (can be found at Negro Cafe locations or order online), Delirante (in Recoleta), Blanca (various locations) and Cuervo Cafe (various locations). For a darker roast, maybe try Full City (Palermo)? They had a bold Colombian bag we used to buy when I first moved here and it was solid and a darker roast than some of the newer cafes are producing. I also just bought an amazing bag of beans from Kissaten Tostador in Belgrano. Lattente (various locations) also have great beans, as does Surry Hills (Las Cañitas).
It really depends on if you're willing to pay extra to bump up quality a bit, many folks understandably aren't in today's economy. I really love coffee, it's one of my little joys in life when all the other joys have priced me out. I don't spend feverishly on anything else, but do pay a bit extra for better coffee beans while I can, until that luxury is also ripped away by 'the market.' haha
Happy hunting!