bike shops are good, i looked on craigslist for a while and finally just gave in and bought a new bike. i´ve enjoyed it since. i went to the shop next to parque lezama in San Telmo. at defensa and caseros.
Dunno how soon you need one, and what your price range is, but if you are looking for a world-class mountain bike and are willing to hold off a few weeks, I am in the process of establishing a distributorship for the Montague line of full-size folding bikes like this one:
After many trials and tribulations with customs, I've finally been able to place my initial order. Pricing will be close to what is offered in the U.S., though a tad higher due to the numerous import duties and high international shipping costs. I will make all details available as soon as they arrive on port.
The initial batch to include all models should be here by late March. Good luck with your search.
I got my mountain bike at Charly A's in Olivos: Av. Libertador 13067 tel: 4792-5084
Nice little place. I think they focus more on mountain bikes but have a nice little selection. Just down the road on the other side of the street (heading into downtown) is TREK - bigger and more expensive models, but larger selection and great quality.