Key has to be coded to the car so the transponder in the key fob is read by the car to allow starting. People did get replacements from BMW that came cut and coded to your car, they did them using records held on your cars build details at the factory. Other option is a mobile locksmith who should be able to read the EWS module which has to be taken from the car and cut and code a replacement. Further option is to send your working key to one of the key suppliers who are Ebay but risky with the post these days.