Read-only access to the sudo source tree (from as far back as 1993) is available for checkout using mercurial at http://www.sudo.ws/repos/sudo. Anonymous CVS access is no longer available.
hg clone http://www.sudo.ws/repos/sudo
This will create a copy of the sudo repo on your local machine in the sudo directory. This step only needs to be done once.
Next, change to the sudo directory that you just created and check out the sudo 1.8 source files.
cd sudo hg update 1.8
This will populate the sudo directory with the sudo source files for the 1.8 branch.
cd sudo hg pull hg update
The hg pull will pull in changes from the master repo, the hg update will update the working copy from your local repo.
You can also do standard operations like log, diff, and annotate. While you can check in changes to your local copy of the repo, you will be unable to push the changes directly upstream. Changes may be submitted in the form of patches or change bundles.
If you are new to mercurial, the mercurial guide is a good place to start.