Nissan should be able to get one cut for you if you give them your VIN. You can try getting yours copied from a regular key place, but be warned they dont always work. I got one copied at a locksmith and it just didnt work for some reason..
Actually kind of a related unrelated story - I locked my keys in the car once, and I got a locksmith to come out and break into it for me. He fashioned a key from OUTSIDE of the car which opened the door, and started the car!!! It took him about 15 minutes. He just stood there with his little portable key grinder, and created a key just from feeling the pins as the key went in and out of the door. It's amazing that he could do that, but the key the locksmith copied from the original didnt work!!