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.