ClassNotFoundException and NoClassDefFoundException are the common errors and also the most irritating errors for a Java developer.The solution for that is you don't have the jar file with you.The best place to look for jar file is findjar.com
The process is very simple..
1) Visit findjar.com
2) Enter a classname or package name and press Jar/Class Search button..
3) If you are not sure about the format in which you should serch you have last queries option below which helps you and guides you the format in which you need to search.
4) The latest advent of their sit is they have added some new functionallity like Eclipse Plug in and Firefox Search Engine Plug in