Did you book with a travel agent, online or with carnival directly? We booked with a TA in the peg & we recieved both transfers with our tickets. Having said that, I have also seen on the Carnival web site that you can purchase the transfers in advance from them.
At the very bottom of this info it says that you can buy the transfer onboard, if you want to wait!
As bluesky and paddletoe53 said, you can purchase the transfers in advance from your TA or your PVP.
Have a wonderful cruise!
"A truly happy person is one who can enjoy the scenery on a detour."