i never got one when i bought my car, how much are they or does anyone have a spare they'd like to sell me !!

Are you talking about the in-the-boot-fitted electronic key?

If so, I wouldn't advise buying one second hand - it won't work. It's touch memory - i.e. there is a memory chip in the key with an unique serial number in it. When you touch it to the port, it transmits the serial number to the alarm. You even try the incorrect one and your alarm will probably go off, rather than silence!

I believe Nissan can re-program them, but I doubt you'll get one from anywhere else.