If you're looking for YuGiO cards, here are a few possible places to buy them online:
Pokeorder carries a wide variety of YuGiO Single Cards, Booster Packs, and Theme Decks.
CrossToy has a more modest collection available, but offers YuGiO booster boxes for sale, which are much cheaper than buying the boosters individually.
Or, check eBay for YuGiO:
YuGiO Video/DVD
Amazon is a good place to find YuGiO videos and DVDs or even books and computer games:
