If you don't live close to a Korean grocery store or even an urban center with a good Asian grocery store, it can be hard to find the right spices and ingredients to cook traditional Korean food. At least, it used to be difficult before the World Wide Web. I've heard stories from Korean-Americans growing up in the Great Plains who took monthly weekend-long bus trips with other Asian families to buy groceries in cities that were many hundreds of miles away. Now you can buy many of your Korean spices, ingredients and already prepared foods online.
My own Korean grocery store, H-Mart has brick and mortar locations in New York, New Jersey, Pennsylvania, Virginia, Maryland, Georgia, Illinois, Texas, Colorado, Nevada, California, and Washington. I love this place because I can fulfill all my Korean food needs and buy fresh pastries, kitchen tools, and massage items all under the same roof. Their online site is just as extensive, with refrigerated Korean food, kimchi, snacks, appliances, cosmetics, and more.
Specializing in instant and packaged Korean goods, Kgrocer also has a full selection of spices, canned foods, and tableware for your kitchen. Their real brick and mortar location is in Los Angeles, where some of the best Korean food in America lives, and I've used them to send care packages to friends starved for Korean food.
An online grocery store selling Chinese, Japanese, and Korean products, Koa Mart has a wide selection of Korean groceries, spices, and sauces. A good selection of kimchi and pickled food can be delivered directly to your door, and they also sell Korean DVDs and charming Asian gifts.