In Charamicos there is a a little shop called Casa de tornillos. He sells nuts and bolts and also cuts keys. He sells transponder keys for a few brands of cars. i know he does Toyota, so I'm fairly sure he will stock blanks for Nissan. He has a coding machine that will clone the transponder chip in your original key. However this is the key only to start the engine.
If you want a key fob to control the central locking, your best bet is to buy one off eBay. There are instructions on the internet on how to program it. It's usually a sequence of operations like switching the ignition on then off several times, followed by opening / closing doors etc.
But again depends on year and model of your Nissan.