I assume if you're reading this you're not going to build your own Android2 and need sources just for reference.
What's the first thing you do to find sources for Google Android? Right, you use that same Google...
Just to find yourself here http://android.git.kernel.org/, where even Google can't help you find anything.
You can invest a few hours of your time setting up the environment downloading everything and searching through it.
You can invest a few hours clicking through the web based interface.
Or you can use this hint. Start from project "platform/frameworks/base.git" found here:
This project and especially subdirectory "core" should contain most of the API related code, which is probably what you've been looking for.
The page contains link "snapshot" which allows you to download the HEAD revision of base.git repository.
Good luck hacking it. :)