1. SCWCD Exam Study Kit: Java Web Component Developer Certification, by Hanumant Deshmukh and Jignesh Malavia. The one book I think you need to study for this certification. Contains a mock exam but I can't comment on it because I lost the CD before I could use it .
2. Free (with a quick mandatory registration) mock exam at http://www.j2eecertificate.com/. This is a superior mock exam site (they also have sites for the SCJP and SCBCD) which goes to show that, like javaranch itself, sometimes the best things in life _are_ free. After completing SCWCD Exam Study Kit, I believe the "learning by testing" strategy at this site is very effective. I suggest studying each section first and then going back and studying any faults. Continue studying the faults until there aren't any more. Then take the difficult exam (20 of the most often missed questions). Then take the final exam. I scored 88% on the final mock exam and 83% on the actual exam.
So, my sincere thanks to Hanumant and Jignesh, and the www.j2eecertificate.com people, who together made studying for the exam a very positive experience.