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