What is the cheapest and fasted way to replace my lost key

You have a few options here. First if you change the ignition cylinder the new one will come with a new key thus if your doors are keyed the same as the ignition now you will have to use 2 keys unless you buy a full lock set for the whole car.

Option 2 is to go to the dealer and see if they can look up your key cut with the VIN and have them make you a new key.

If they can't do that I would try a good lock smith. You will have to remove the key cylinder and give it to them and see if they can make you a key but I would call them first and ask if they can do it.

Let us know what you find out.

Rich