Eyes and Beholders: WoW Battle Pets
Select a pet:

Quest: Darkmoon Pet Battle! [1]
Zone: Darkmoon Island
Drop: Darkmoon Pet Supplies
Approximate Drop Rate: 1 in 56

Vendor: First Arcanist Thalyssra
Zone: Suramar
Faction: The Nightfallen - Revered
Cost: 500 (unlimited supply)

Drop: Demon Boss
Zone: Dreadscar Rift (Warlock Order Hall), The Fel Hammer (Demon Hunter Order Hall)
Approximate Drop Rate: Unknown
TIP: You can filter pets in a wide variety of ways using the pet filters. You can even apply multiple filters to really zero in what you're seeking.