When i bought my car from a non-Alfa dealer I bought it conditional on getting the dark red master key, plus a spare, which they didnt have. They bought it from the Alfa dealer (probably was a p-ex car that was too tired for the main dealer to sell).
The Alfa dealer had lost the key so they programmed a new one - they had to get a special guy in to do it, that works around the region, presumably for Alfa. I now have the three keys, though the new spare won't (and cannot) be programmed to use the central locking and alarm. (The dark red master key is the most important, and is needed if the ecu loses its memory, i think, so you can start the car - i'm a bit fuzzy on this but someone else will fill in i expect).
But as for codes, they are only available if the original dealer (when sold new) bothered to enter them on the database, in my case they had not, but at least i got a reasonable solution.
The codes were never supplied to owners in the UK at the request of the insurance industry, so its this database, or nothing. You could approach the original dealer (look in the service book).
The "key guy" said there was no other solution, and suggested that if i lost the one working key maybe fitting a replacement Fiat alarm would be an idea - it would mean an extra thingy on the key fob, but it would work the central locking and alarm.
The other fix is a new ecu and keys which i gather is about £1000!!!
Good luck with it
I did a web search and there were some companies that could copy the codes from keys to make duplicates - but of course you need a working original, and when i looked they weren't doing Alfa - maybe they are now.