Lil’ XT, along with 2 other battle pets, were recently “retired” from the Blizzard Store.
https://news.blizzard.com/en-us/world-of-warcraft/22839054/save-big-on-pets-mounts-helms-and-more-during-our-holiday-sale
We’re sending nine of our Blizzard Shop and in-game store items into hibernation* on January 6, 2019
Bring Lil’ XT, Moonkin Hatchling (both Horde and Alliance version), and Cenarion Hatchling home to your collection.
*Items in the See You Later Bundle may return to the Blizzard Shop and in-game shop at some point in the future.
So no longer available at the moment, but it might return at an unknown date in the future.
Edited with quotes from the Blizzard announcement.