Have you ever started a pattern and realized you didn't buy enough yarn? Than off to the store you go and find out that it was discontinued? I have and that is really discouraging.
But don't give up because there are still a few places you can check and see if you can get more. One is the merchants below, another would be try ebay and see if someone is selling off their left over yarn.
A very helpful place is crochetville. There are lots of folks there and they often post for just this reason.
So remember to buy all the yarn you need when you first purchase the materials. I always buy one extra skein, just in case.