Redakai cards are not widely available as of the time of this post. It was well known that Spin Master, the company behind the already popular card game, was only going to release the game to 250 select hobby shops before distributing to larger retail outlets.
As of June 4th, according to the Redakai news, you will start to hear and read about local hobby shops having their “release day events”. These include card games and giveaways across the United States. Just doing a quick Google Search and I see there are plenty of online hobby shops, and Amazon.com, are places you can get sealed game packs, etc.
Don’t forget about accessories that you can buy that will make your game easy to setup and clean up. There is also things you can buy to keep your cards organized for league play, etc.