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.
"If you don't know where you are going, any road will get you there!"