There's a difference between the fob and the key. If you have only one security key, you'll need a locksmith or the dealer to make a working security key. If you have two security keys, you can add one yourself.
If all you need is a fob, you can get them off eBay for $15-$30 (make sure the FCC number matches the existing fob). Then you can use the working fob to add up to three more (I believe - that's for my '03). I have four working fobs, but I don't have the security system, thankfully, so the $2 from Lowe's is sufficient.
In either event, the O/M will show you how to proceed.