You can try fleabay, but Caveat Emptor. You have to make sure it is the correct one for your car. I haven't had the need for one, and I don't know if the stealership has to program it in any way. There has been some posts in the past for this topic, so maybe a little search might help you out.
get em off ebay, it should look exactly like yours but most of the ebay listings say what's compatible in a spreadsheet
it's like $50, you can program them yourself easily, it's even in the instruction manual, you don't have to go to the dealer at all. I think they charge like $200 for the key and $60 to program it? what a racket.
be away that with the keyless access models, there are some subtleties to programming them, like keeping the other keys out of range or something. do a forum search for some topics on the matter