You dont NEED a Fizik seatpost, you just need a seatpost which has been created with carbon seat rails in mind (they are usually larger in diameter or oval shaped) and many compatible seatposts exist. You also need to make sure the clamp contact points are not offset, the contact points must be adjacent, if they are offset, you will end up with a mess on your hands.
You could either return the saddle and buy a different one, or you could choose a few seat posts you like, then drop
Wiggle an email and (assuming they have the same saddle to hand) they will check if those seatpost's will work with that saddle. They have done it for me in the past.