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