WarcraftPets provides you with a bunch of resources for building your collection of companion pets. In addition to listing every companion pet
in-game, we also offer a guide to collecting Vanity Pets by Zone
and you can filter pets
in a variety of useful ways.
Here are some other resources that are worth a look...
Wowhead's Non-combat Pet Guide
Perculia of Wowheadnews.com wrote up a wonderful, compact overview of all our favorite mini-companions.
features a complete list of pets as well as pet-related achievements. Wowhead's article has a dedicated section that also lists by source, such as pets earned from quests, trading card game companions, and many others. In addition to our site, it's a great starter resource, especially for collectors who are just beginning the awesome adventure of collecting pets!
The Undermine Journal
If you've never heard of The Undermine Journal
before, it's a website that tracks, lists, and alerts you of current and updated in-game auctions even if you're not in-game!
Each (US) realm has its own section with auction house categories such as Hot Items, Enhancements and Consumables. A similar EU dedicated website link can be found on The Undermine Journal's front page.
It has been brought to our attention that this informational website has added a companions category
(link shows all pets on the Alliance Scarlet Crusade realm) in the Enhancement section. So if you're looking to start playing the vanity pet market or just seeking a cheap and quick purchase, The Undermine Journal is a great resource.
Good luck and Happy Collecting!