You CAN NOT code a second hand Key if you have the black flip type key. The immobiliser chip can only be coded once and never coded again and you can not but Alfa Romeo immobiliser chips from anywhere. Even auto locksmiths cant get them. Your only option is to go to a main dealer and buy a new key.
Please don't listen to the fairy tales you will hear on here, You will be throwing money away getting second hand keys and a blank key blade and trying and getting it coded. It can not be done.
Ring round loads of different dealers for a quote on a key as prices do differ and get an indi with a Alfa/Fiat diag tool to code the key for you that is if you have the key code. If not then that will cost you a further £20 from Alfa Romeo.
Leader of the NORM collective.
The group against the deformation of our beautifull Alfa's.
Barry boys your days are numbered.