Mi pileta is a manufacturer, not a custom shop-
http://www.mipileta.com.ar/espanol/empresa.php
Franke is another good manufacturer-
http://www.franke.com/countries/ar/es/home.html
personally, I think Franke is higher quality than Mi Pileta, its the Argentine branch of a Swiss company, and they make fewer models, but they are quite well made. They have a showroom down on Santa Fe and Nove de Julio.
the problem with a custom sink is that it must be fabricated, not stamped.
When manufacturing stainless steel sinks in quantity, like both Mi Pileta and Franke do, the cost of a custom die set (easily $50,000 or more, US dollars) is amortized over thousands, or tens of thousands, of sinks.
Of course, it is economically ridiculous to make a custom die for ONE sink.
So, one offs are cut and paste, from flat sheet, and then welded at all the seams. They can be round, as the flat sheet can be rolled pretty easily, but 3D compound curves, like the ones in the mass produced sinks, are very very difficult.
I do some raising (the technical term for 3D shaping) of stainless steel, and, by hand, one at a time, this is a very difficult process. I really doubt you will find a shop in Argentina capable of this in stainless.
So- custom, you would be stuck with a pretty clunky shape, basically one that you can create from flat sheets of cardboard and tape. In fact, if you decide to go this route, that is what I would suggest you do first- make a full size model of exactly what you want, from cardboard.
The problem is, the welds will be somewhat visible, even if done by a really good shop.
How custom do you need?
The best way to go is to find a suitable size sink, that can be fit by just cutting away some of the flat upper area, rather than complete custom building one.
Or, perhaps, go with brass or bronze, which is much more possible to hand form to a custom shape.
Stainless is very hard to work with, and expensive to do right. I do a lot of stainless fabrication, and my rule of thumb is that materials are five times steel, and labor triple. And I have a huge amount of expensive equipment- more than the average shop in Buenos Aires does, thats for sure.
If you really want and need custom, in stainless, I would recommend going and talking to these guys-
http://www.caldinox.com.ar/
they are a big full line stainless supplier TO shops like the one you want to find, and, generally, suppliers like this know who is good and who isnt.
They arent that far away, in a surprisingly urban area near Villa Crespo.
You would need to go in person- email or phone just wouldnt work for this kind of request. I do this kind of thing all the time, tracking down suppliers, and you have to go in, apologize for your crummy Castellano, and then, basically, dont take no for an answer. Dont be put off by the first salesman, ask to talk to the bosses, and chances are, they will find you a shop that can do it.
Dont expect it to be cheap, though. Stainless alone for a custom sink will easily run a few hundred bucks, and, with labor and an industry standard pain in the ass charge, depending on size, this could easily be a $500 to $1000 dollar job.
If its not huge, you might consider having it made in the USA, and taking it back in checked luggage. I have done this with several custom components for my departmento, including some stainless furniture pieces like these little footstools...