Magical Pet Biscuit

Magical Pet Biscuits are items that can be used to double the size of the selected companion.

Source
Magical Pet Biscuits can sometimes be found in Sack of Pet Supplies. They are also granted in stacks of 25 as rewards from a number of quests from Children's Week:


 * A Warden of the Alliance/A Warden of the Horde
 * Back To The Orphanage (Oracles)/Back To The Orphanage (Frenzyheart)
 * Back to the Orphanage

Additionally, Whizzig at the Celestial Court on the Timeless Isle sells a stack of 25 in exchange for.