Welcome to planDisney, Erin! I'm so excited to hear that you're going to be at Walt Disney World Resort next week. This is such a fun and unique question, and I did some digging to get to the bottom of it for you, so let's jump right in and chat about what I found out!
Erin, I got in contact with some Cast Members who work with merchandise, and they told me that the mailable coconuts come from a third-party vendor, so their availability varies. While we can't say for sure if you'll find them during your trip this week, I can say with certainty that if they were to be available, you would be able to purchase them at
Plaza del Sol Caribe Bazaar near
Pirates of the Caribbean at
Magic Kingdom Park,
BouTiki at
Disney's Polynesian Village Resort, or
Calypso Trading Post at
Disney's Caribbean Beach Resort. These are some of the most popular shopping destinations because they're easy to get to! Magic Kingdom Park and Disney's Polynesian Village Resort are both on the
monorail line, and Disney's Caribbean Beach Resort is accessible by the
Disney Skyliner, so it's highly likely that you'll be passing by these locations at some point during your trip.
If there aren't any coconuts to send through the mail, maybe your nephew would want to send his dad some mail directly from Main Street, U.S.A.! Stop by the Main Street Chamber of Commerce to get the special "Mailed from Main Street" stamp and drop your letter or postcard in one of the post boxes around Magic Kingdom Park. It will be such a fun souvenir for your nephew and his dad!
I hope this answers your question, Erin, and please let us know if there's anything else we can do to help you plan your magical vacation.
Jessica