Once you’ve completed the initial set of battles, the next series of Battle Pet Tamers become available from Audrey Burnhep in Stormwind. This is an account-wide but faction specific quest.

Defeating each one will give you a 5min Pet Recovery debuff, meaning you’ll have to wait out the 5min before you can battle another Battle Pet Tamer.

The five you will need to defeat in Eastern Kingdoms are:

David KosseDavid Kosse <Master Pet Tamer>
Location: The Hinderlands – Stands just off the path between the forked part of the road and Jintha’alor
Level 13 Pets: Subject 142 (Critter), Plop (Magic) and Corpsefeeder (Beast)

Deiza PlaguehornDeiza Plaguehorn <Master Pet Tamer>
Location: Eastern Plaguelands – Other side of the lake at Light’s Hope Chapel
Level 14 Pets: Plaguebringer (Undead), Bleakspinner (Beast) and Carrion (Critter)

Kortas DarkhammerKortas Darkhammer <Master Pet Tamer>
Location: Searing Gorge – next to the Horde flight master in Thorium Point
Level 15 Pets: Garnestrasz, Veridia and Obsidion (3x Dragonkin)

EveressaEveressa <Master Pet Tamer>
Location: Swamp of Sorrows – Between Marshtide Watch and Sorrowmurk
Level 16 Pets: Dampwing (Flying), Croaker (Aquatic), Anklor (Critter)

Durin DarkhammerDurin Darkhammer <Master Pet Tamer>
Location: Burning Steppes – at the base of Blackrock Mountain
Level 17 Pets: Ignious (Critter), Comet (Flying) and Moltar (Elemental)

Completing the second tier of Battle Pet Tamers will open up another quest – Grand Master Lydia Accoste.

Lydia AccosteLydia Accoste <Grand Master Pet Tamer>
Location: Karazhan – Between the church and the graveyard
Level 19 Elite Pets: Bishibosh (Undead), Nightstalker (Undead), Jack (Elemental)

Defeating the Grand Master on each continent will unlock daily quests (via an achievement) for all of the other Tamers on that same continent (10 shots a day if you do them all). In fact, you can battle Lydia again as she becomes a daily.

Completing this chain allows access to the Outland series of battles.