My first guide, so feel free to rip me up afterwords
This guide is for all vendor-sold pets, usually betweem 10 silver to several gold, and a few for a good chunk of gold.
You will need to get several other pets besides these after you get them all, so i suggest going to "Warcraftpets.com" and seeing what sounds up your alley to get.
Ancona Chicken - Magus Tirth - Thousand Needles
Black Kingsnake/Brown Snake/Crimson Snake - Xan'tish - Orgrimmar
Blue Dragonhawk Hatchling/Brown Rabbit/Mana Wyrmling/Red Moth - Dealer Rashaad - Netherstorm
Blue Moth/Yellow Moth/White Moth - Sixx - Exodar
Bombay Cat/Cornish Rex Cat/Orange Tabby Cat/Silver Tabby Cat - Donni Anthania - Elwynn Forest.
Brown Prairie Dog - Halpa - Thunder Bluff
Cockatiel/Senegal - Narkk - Booty Bay
Cockroach - Jeremiah Payson - Undercity
Golden Dragonhawk Hatchling/Silver Dragonhawk Hatchling/Red Dragonhawk Hatchling - Jilanne - Eversong Woods
Great Horned Owl/Hawk Owl - Shylenai - Darnassus
Nether Ray Fry - Grella - Skettis (Note - This pet requires exalted status with sha'tari skyguard reputation.)
Snowshoe Rabbit - Yarlyn Amberstill - Dun Morogh
Spirit of Summer - Midsummer Merchant (Note - only available during the Midsummer Fire Festival World Event.)
Tree Frog/Wood Frog - Flik - Darkmoon Faire (Once a month - Check calender for times.)
Wolpertinger - Belbi Quikswitch/Blix Fixwidget - Durotar/Dun Morogh (Note - Only available during Brewfest Event.)
Tiny Sporebat - Mycah - Zangarmarsh (Note - Requires exalted status with the Sporeggar reputation.)
Buying all of these pets will put you at 31/50 pets. You will be required to get more for the 50 pet achievement and the pet skunk "Stinker". For more pets and details on them, check Warcraftpets.com. Good luck, and happy hunting!