The problem with replacement keys/fobs is that they need programming into the security computer on the car.
I think the cheapest way to get a key is a company that sends you out a key and a box of tricks that plugs into the diagnostics port on the car to program it. You then return the box of tricks for a deposit back. I don't know how they get over the physical aspects of the barrel and whether this includes the alarm/remote locking features or just the immobiliser. Google is your friend to find this company and exactly what they provide. As said, it may also depend on the year of your car.