I don't think there's a Moodle Administrator Certificate yet. I suppose when there is one it will be mentioned in the official certification area: http://certificates.moodle.com/
I obtained my course creator certification (MCCC) by taking a course with HRDNZ. They also have courses for administrators: http://www.moodlebites.com/course/index.php?categoryid=3. But I think these are only courses are don't lead to official certification. I had a great learning experience with HRDNZ so I strongly recommend them.