Go to https://www.mastersommeliers.org/student and login to your account.
Go to My Account and Click on "Order Replacement Pin." They will ship after verification. As of 2019, the intro pin was $10.
You can email Kathleen Lewis (email@example.com) if you've lost your login information for the CMS website.
I hope this helps anyone who has lost a pin on a busy night!
To help with them falling off you can cut a piece of cork to use as a backing instead. They also sell locking pin backs all over the internet.