I had a very large painting stretched and custom-framed at Inquadrare at Arenales 3638, between Julian Alvarez and Araoz. They did a nice job, although as expected, it took some weeks to do it (including a couple weeks more than they had promised).
Before selecting them, I checked 4 or 5 other framing places in the neighborhood. I found the prices to be all about the same, but I liked the work on display at Inquadrare a little better than the others.
I work a fair amount with a couple of guys who run a little shop called Carpe Diem, on Guemes between Billinghurst y Bustamante.
Good guys, honest and pretty quick, and reasonably priced.
Not museum quality for $15 Million dollar old masters, but for most art, they do a great job.
In Buenos Aires, most frame shops dont stock any frame material- they order it all in. So you have to expect that it will take a few days to get in your framestock, or cut glass or plex a custom size.