Hello Jane (cute name by the way!) and a warm welcome to the Disney Parks Moms Panel.
Firstly I must say that as the current British Mum here on the Moms Panel it's always an added delight for me to offer some pixie dusted assistance to a fellow Guest who also hails from the United Kingdom!
I know just exactly the coconut postcards you're talking about Jane as I've seen them myself on past Disney holidays and have joked with my family about how fun would it be to receive one through the post direct from Walt Disney World Resort.
Disney Resort Guests benefit from the convenience of
Mail Services at various select locations on Disney property for both sending and receiving of post although I'm pretty sure that our local postman here in Cornwall would have quite the tussle making a whole coconut fit through our letter box!
Whilst I can't guarantee the current availability of these totally nutty novelties I can share that during previous Disney holidays I've seen them displayed and available to purchase in the shops at
Disney's Caribbean Beach Resort and in
BouTiki at Disney's Polynesian Village Resort and in
Plaza del Sol Caribe Bazaar within the Magic Kingdom Park.
Magical wishes!
~ Jane