I just recently was helping a couple that was looking for a longer-term rental. My function was strictly as a translator, but I learned a lot about the apartment rental market in Buenos Aires. Here is my perspective: For a
furnished apartment, the
longest term a standard contract (one that can stand up in court) that can be done is for only 6 months.
For an
unfurnished apartment the
shortest term that can be done is 2 years and you have to have the said
guarantia. Which means you have to get an Argentinian property owner or bank to co-sign your rent agreement. It is difficult to get a local owner to even consider renting you an unfurnished apartment for less than 2 years and without a guarantia. If you are here on your own (ie. not on an ex-pat contract with the back up of a large well known company) it is very difficult to get a guarantia.
We found that many owners (especially overseas owners) that rent a furnished place, often like to use it themselves or for family members through out the year. Meaning that they were not that interested in 3-6 month rentals. Especially now as many come during the Christmas season. If you are working through an agency, the nice apartments tend to be rented out a few months out so you would have to find an apartment that has no one renting for the next 3-6 months. Maybe a newer listing, for example.
Many have found their way around some of these obstacles by talking directly to the owners of apartments. But how does one do that? It is mostly through word of mouth. Once you get here and get your network established you find someone, that knows someone etc.
In the end I managed to hook up this couple with the owner of my apartment that happened to have a second apartment which he had not yet listed with any agency and therefore was free for the next 6 months. As they worked directly with him they also got a good price. By the way I met this couple just a few weeks ago at one of Dave's BBQ's
http://baexpats.org/expat-events/5092-burgers-burbs-6-14-a.html
This is a great place to meet and socialize. So call it luck, fate, coincidence, connecting, networking - whatever you want to call it - that's how they got their rental.
Just know that the rental market and rental contracts work very different here than in the USA. The mentality of a local owner is distrust of everyone as the laws here favor rentors not owners. If you try and approach it like to US rental market you will be very frustrated.