The Thundering Serpent Hatchling isn't the easiest pet to find. In fact, you don't really find IT specifically. Instead, you need to find a guild who's unlocked it on their guild vendor. Even then, guilds that have put the time and effort into the achievement that unlocks it, may not be open to inviting anyone and everyone seeking the pet.
That being said, there's good news for all collectors still seeking the Thundering Serpent Hatchling! Doobjanka (Community Leader and fellow admin of WarcraftPets) now has a Horde guild on the US server Madoran called Magical Menagerie. His guild has the serpent companion unlocked, and all are welcome to join and earn it. :) Simply message any member of Magical Menagerie for an invite.
You'll need to reach revered status with the guild and have 270 gold to purchase it, but it's not that difficult (trust me, I've done it before :P).
Other than that, it's great that collectors now have a guaranteed guild to go to for this pet (and other companions). Just keep in mind that although the guild is open for all seeking the pet(s), general social etiquette should still be practiced. Follow the guild rules and keep things friendly. :)
For those that aren't on US servers, WarcraftPets has a forum thread that lists potential guilds that are open to allowing players to join for the Thundering Serpent Hatchling.
Many thanks to Doobjanka for acquiring the guild and opening it up to collectors. :)