what BMW model and built year do you have? With EWS with the chip inside or without?
If without, just buy on the auction house *bay a BMW key blank and bring it to a locksmith specialized on car keys to make a copy. But of course you still have to have a spare.
Ot find the next car locksmith in your area Find a Locksmith
If no spare to make a copy, the fastest is your local BMW dealer, he can order one from BMW to git your doorlocks and ignition.