If you originally checked out the code using
cvs -z3 -d:pserver:anonymous@SERVER.cvs.moodle.org:/cvsroot/moodle co -r MOODLE_18_STABLE moodle
I think you can then check out a module from contrib using:
cd moodle
cvs co -r MOODLE_18_STABLE -d mod/CUSTOMMOD contrib/mod/CUSTOMMOD
Where you should change CUSTOMMOD to the name of the module you want (or change of the path if you wanted to check out a quesion type, say).
I have not actually tested this, I just know how it is supposed to work. If you try it and can confirm it works, please add this to
http://docs.moodle.org/en/CVS_for_Administrators.
After you have checked out something like this, a single CVS update in the top level folder should update Moodle and all plugins.