You can get them from Outer Limits in Melville. Or try Pops Games in the Northmead Square.
However, if you are serious about getting into a competitive trading card game, you should check out: Magic: the Gathering on www.magicthegathering.com
There are a bunch of places all over the country that host organised tournaments. There are about 800 or so active players in the country. There will be a huge release tournament held at Rage during the first weekend of October.
You could get a game tutorial of the website explaining how it works. It's been around a lot longer than the WOW card game and it is continuously updated and tweaked.
i havent checked but i know they sell magic etc in bt games. There is also a big fantasy shop in edenvale where they used to sell all the trading card thingies ... dunno if they are still around