A couple of points to start with:
1. There wouldn't be much point in radios having security codes if every radio in a particular model of car had the same code. Every radio has an
individual code.
2. The Answerbank has a policy of not allowing radio codes to be passed on via this site, as that could assist criminals.
So you're going to have to fork out some money to get hold of the code. The cheapest way is likely to be using a website such as this one (which hasa good reputation):
https://www.radio-code.co.uk
However there's a potential problem in doing so, in that you need to remove the radio from its mount to read its serial number and, without special tools, that can turn out to be extremely difficult!
Alternatively, any Nissan main dealer should be able to get the code directly from Nissan. That can be done simply be reading the chassis number of the vehicle, so there's no need to remove the radio. (When I worked for a Ford main dealer, a few years ago, the charge for providing a radio code was £15, so it's likely that Nissan would make a similar charge).
A final point though: Have you checked whether someone has written the radio code on the cover of (or inside the cover of) the manual for the radio (or the one for the car)? Simply taking a look in your glovebox might save you some money!