Hi Melissa! Welcome aboard! It's almost time for your Disney cruise on the Fantasy! I bet you are very excited!
If you order today with fast shipping, you could get the
Disney Cruise Line autograph book from the Disney store online just in time for your sailing.
I've not seen the Disney Cruise Line specific autograph books at the parks recently, though merchandise is always subject to change, so that's not to say they aren't available somewhere at the Walt Disney World resort.
But, honestly, once you board the ship, there won't be many character greetings until you set sail, and once that happens, the stores onboard will open, usually around 6:15 or so. Mickey's Mates carries Disney Cruise Line autograph books, so you could pick one up there before you head off to explore the ship and find some characters that first evening. One less thing to pack as you get on the ship, and most likely you'll find yourself exploring the shops onboard that first evening anyway.
Have a great cruise, Melissa!