When I was down (just shortly) one key in the spare set from the rental agency I used didn't work, so I just took a walk in the barrio I was staying (Palermo -- Mansilla/Bulnes) and went around until someone whipped me off a copy.
Lots of stores -- finding one shouldn't be a problem. Doesn't necessarily need to be a cerrajeria either.
The one key I got cut was 8 pesos.
No questions asked.
This would have been a lot quicker than mentioning it to the agency..