Battle-Training Stone

Battle-Training Stones are items used to add 1 level to a battle pet.

Ultimate Battle-Training Stone can be used to upgrade a pet to level 25, while all kinds of Battle-Training Stone will upgrade a pet by 1 level. Ultimate Battle-Training Stone and the generic Flawless Battle-Training Stone can be used on any pet, while all other kinds can only be used on a pet of the appropriate type.

Source
One Type-specific Battle-Training Stone of any kind is contained in Mystery Bag, sold by battle pet master in your garrison.

Flawless Battle-Training Stone are sold by battle pet master in your garrison.

Level 100 characters with a Tier-3 Garrison can complete a quest to receive an Ultimate Battle-Training Stone to increase a single Battle Pet to level 25..

Battle-Stones

 * Flawless Battle-Training Stone
 * Fel-Touched Battle-Training Stone
 * Aquatic Battle-Training Stone
 * Beast Battle-Training Stone
 * Critter Battle-Training Stone
 * Dragonkin Battle-Training Stone
 * Elemental Battle-Training Stone
 * Flying Battle-Training Stone
 * Humanoid Battle-Training Stone
 * Magic Battle-Training Stone
 * Mechanical Battle-Training Stone
 * Undead Battle-Training Stone
 * Ultimate Battle-Training Stone