You mean Keyless Entry fob?
Well first you have to determine that your car has the hardware already installed--that is, that it was ordered with Keyless Entry.
If it does have that option, there are quite a few sites online that sell refurbished remotes, or you can buy from the dealer. Be sure you get proper programming instructions.
Here's one website that sells 'em, but I'm not recommending them. I don't know anything about these people.
If you don't have this option already on your car, you can get an aftermarket installation from any good Car Audio shop.