If you are close to Palermo, try this place:
https://www.google.com/maps/@-34.5781224,-58.4276561,3a,75y,133.95h,97.34t/data=!3m4!1e1!3m2!1s6J2HRJ-DmmhpE04XH6y8_Q!2e0
As a previous poster said, FV is your best bet. High quality and available, since they are domestic. They are not cheap, however. I recommend the place above because they will often sell individual parts, without having to by an entire set (handles, faucet, shower head).
There are a lot of plumbing places on Junin near Corrientes (try between Tucuman and Peron), but none of them sold individual parts when I was looking late last year.