Missing components in Eagle Library
I would recommend, that you learn how you create your parts on your own.
For me it is often faster than searching for a missing part. When you would find the part on the net you should check it for correct pin out, size, etc.
Here is a tutorial I found quite useful.
However there are some places where you can find libraries:

User created libraries at cadsoft
Sparkfun Eagle Library


Let me add a note about creating your own part:
When your part is in a common package format, you should take a look at the ref-packages.lbr library. It contains various standard package formats, so you dont have to create these on your own. Just copy the needed package in your own library and create the rest according to the tutorials you will find on the net.