The previous posting (now deleted) had bad URLs with a "..." in them. I am SURE that I tested them after posting. They seem to have changed.
Anyway, here are the full correct URLs:
It took a bit of searching, but here is the new source code download page:
http://www.amazon.com/gp/help/custom...deId=200203720
And the links to the moved source code (until they get moved again):
4.0:
http://kindle.s3.amazonaws.com/Kindl...8590058.tar.gz
4.0.1:
http://kindle.s3.amazonaws.com/Kindl...1440003.tar.gz
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5.0:
http://kindle.s3.amazonaws.com/Kindl...0280073.tar.gz
5.1.0:
http://s3.amazonaws.com/kindle/Kindl...7760049.tar.gz