Just out of interest, what and how many do you get when you buy a new car? (I have never bought a new car - I like old ones).
As far as I know, you need the number or the master key, and you take it to the dealership, they will order one, it's pretty expensive... some elecronic keys cost around £120-£150 + vat - and i think thats the trade price.
Some manufacturers keep all the details of a car on computer so you only need the registration for them to look up the right key, but of course you will need proof of ownership.
It can take 1-2 weeks to get a key from time of ordering.