Here are links to official sites and elsewhere.

Official Sites

Nickelodeon's official Avatar website
Nickelodeon's English Avatar homebase

Upper Deck Entertainment
Makers of the Avatar TCG

Vuduberi Studios
The artists who drew the Avatar TCG art
Also drew the Avatar TCG art

Also drew the Avatar TCG art

Dan Panosian
Drew the Burger King Avatar promo cards art

Drew the "Ice Attack" & "Air Cannon" Aang toy promo cards art

Sean Galloway
Drew the "Fire Blast Zuko" toy promo card art


Where you can buy the TCG

The game is no longer in production, but you can buy it second-hand
Starter Deck 1   Starter Deck 2   Booster Packs
There are still many on sale
You may find individual cards or sets on sale


General Avatar

Wikipedia // Avatar Wikia
Very in-depth info on the show, influences, media, and much more

Wikipedia's Avatar TCG page
Info on the game, trivia, and more

Distant Horizon
General & in-depth Avatar info, graphics, sprites, and more
News, interviews, episode transcripts, screenshots, and more

Dongbu Feng
Show and inside-source info, podcasts, and more

LiveJournal: Avatar Fans
A large community for all things Avatar

LiveJournal: Avatar Cards
A quiet community for fans of the Avatar TCG