There are so many sites out there that are good and more out every week it seems, but I agree that Lion Brand is good. I am very picky about organization and looks etc. and I think Lion Brand is the best organized of all the yarn sites....of course I go to Colinette.com frequently to dream but they do not sell direct in the US and their yarn is so expensive. Almost every yarn manufacturer has a site but not all of them sell the yarn as well.....however, they usually provide you with a way to look up a local retail store that carries their yarn.
Don't forget that this site can't be beat for information and organization and is always a great place to start looking for what every you need because not only is it organized well and has the wonderful videos but everyone is so helpful in the forum.