Online LOTRO card sellers:

• Elf Magic Cards - my current favourite online seller. Always around when I want to buy cards, normally has 4 copies of each card, and great prices (with 20% off if you spend more than $10)
• Daplummah - has been around longer the Elf Magic and some prices are incredibly low, but he's not available as often in the Trade Lobby. Worth grabbing if you get the chance though!

These places are great for decks and articles:

• LOTRTCG.fanhhq - now the official site for LOTR TCG
• CobraCards - an offline card seller, but has active forums and strategy article contest
• Decktech.net - news, articles, deck designs
• GamePlayersNetwork.com - forums, articles, deck designs

I also find these LOTRTCG links quite useful. Maybe you will too...

• Lord of the Rings Online TCG Forums - home page for LOTROTCG
• Unofficial Errata for LOTR TCG - my friend Kev-La and I use this unofficial errata to play x-listed cards in Movie Block Standard games together.
• Decipher's LOTRTCG BBS - Decipher's Lord of the Rings message boards
• LOTROTCG Blog - my good friend Grashopper's weblog about LOTROTCG
• TradingCardBrowser.com - browse and search all LOTR TCG cards - mighty useful for deckbuilding!