Obtaining the library source from Mercurial
From CEGUI Wiki - Crazy Eddie's GUI System (Open Source)
To obtain a copy of the code from mercurial, you might use the following commands (or thier equivalent, if you use a GUI based mercurial client)
hg clone https://bitbucket.org/cegui/cegui cegui-source
Once you have this, the cloned repository is updated to the default (unstable) code, so you most likely will want to switch to the stable branch instead (be aware that the -C option here will discard any local file changes without additional warning):
hg update -C v0-8
You can also browse the repository from your web browser here: https://bitbucket.org/cegui/cegui/src/