You can get a replacement key made by either getting the keycard from fiat and a autolocksmith should be able to cut the key from this else you could have the lock decoded by a tool or by opening a lock and cutting from the pins in the lock, else its a fiat job.
Once you've got a key cut you'd still need it programming.
If you go to fiat they'll do it all but will cost £££££
You could buy a replacement ecu, fusebox and lockset from ebay, which would be the cheapest option (make sure theres 2 keys) although you will need to remove the ignition lock for this which wont be too easy without the proper key. So you'd have to find a way to remove without a key or get a key made up just to remove the lock.