Cerrajeria means lock shop and a cerradura is a lock.
Llaves are keys (feminine)
They are numerous in Palermo, Belgrano, etc.
You can also get copies made at Jumbo (usually opposite the checkouts).
The "classic" solid brass copies should be no more than 5 pesos.
Security keys (typically for the front door of an apartment building) should cost no more than 15 each, but only for the "delux" with plastic over brass on the end.
I recently had "delux" copies made at a cost of 14 pesos each for an expat friend who doesn't speak castellano. He was quote 30 pesos each at the same cerrajeria.