You would probably have to order it from the dealership, the transponder in the key is tied to the car.
Not sure how much they are for the 1 series, but for an E46 I think it was around $450, that's with the locking remote.
If you wanted a spare key just to be able to start the car, get a quote for a valet key, it will open the door and start the engine, but it won't lock / unlock the boot or glove box, and also doesn't have the remote features.
When I got a valet key for my car (E46) it was $180, ordered via Team McMillan.
I am be possibly wrong in regards with info for your car, but maybe some of the other members may be able to shed some light on this.