I've been in Buenos Aires two years ago, and finding rent boy is very easy if you know where to look for them. Needless to say if you compare Buenos Aires to Rio or São Paulo, Brazil wins by far.
The rent boys are in Lavalle street and in Santa Fe Avenue. As @msclelovr said, you have to be very careful because there is a lot of crime and many underage hustlers, but they are abundant and very cheap.
The bar in the basement is called Contramano and it is the senior (and seniors') gay bar in Buenos Aires. It has never been an escorts bar, but a venue with a crowd older in average than other places. During my last years in Buenos Aires, when I was in my 30s, I was very successful picking up very young guys there, besides the United Airlines flight attendants that I would often meet there. Perhaps some of the guys I was hooking up with for free would charge to older guys, I do not know. But in the local gay culture the place was never considered a hookers bar, but and older gentlemen' gathering venue.
It is not located on Santa Fe but on Rodriguez Peña, a few meters from Santa Fe. It is not just a basement, about 20 years ago they expanded with a more modern three floors wing. In my last visits I did not go there because I am not willing to switch to the crazy night life times of the locals. Going to the place before 2 AM is pointless, and it is not an after-hours, 2 AM is the time when porteños go out, after a late diner.
I did explore the porn movie theaters (a big one called ABC, but I think it was closed), the street cruising (still abundant although not as much as 20 years ago), and of course www.soytuyo.com. You probably have seen my earlier post recommending Joel.