It looks like there might be a new achievement in patch 4.1 for players who collect 100 or 125 pets. I notice that just the rumor of this has increased the amount of vendor vanity pets that I've been selling on the auction house the last two weeks. So if you need some extra gold, now would be a good time to do a little running around and grab a bunch of these little fellows to post on the AH. The best thing is most of these you can buy for less then a gold from the vendor and sell for 10 to 100 gold on the AH. I would post them right away since they seem to be selling well. Plus you'll make some gold now just incase the achievement isn't actually coming out in the next patch like its rumored to be. If the achievement is in the patch you can expect a rush for these the first week or so. I'll give you a list of the easier pets that you can get without having to travel to far.
Vender Pets in or near Alliance Cities:
Darnassus: seller-Shylenai,
Elwynn Forest: seller-Donni Anthania
Exodar: seller-Sixx
Vender Pets in Horde Cities:
Under City: seller-Jememiah Dayson
Thunder Bluff: seller-Holpa
Orgimmar: seller-Xan'Tish
Other Locations for pets:
Booty Bay: seller-Narkk
The Stormspire: seller Dealer Rashaad
Dalaran: seller-Breanni
The prices I listed for the vendor pets might be a little different for you depending on the reputation you have with certain factions.
There are pets you can buy with your Champion's Seals in Icecrown
There are still some more vendor pets scattered around that I didn't list. Engineers and Enchanters can each make pets and there are a lot that drop off of certain mobs. I hope this helps you out whether your going for the achievement or trying to make some extra gold. There are many other gold making methods that can help you increase your gaming experience. You just need to know were to look for them.