Hi,
Look in the jars in the C:\Documentum\cache\<version>\bof\<repository> folder.
You can find the class file for example with total commander (search in archives).
Search on the jar's name in the content.xml in the same folder. The file's parent "entry" node has an object name, in this case com.documentum.services.dam.df.IStoryboardService.
So look in the /System/Modules/SBO/com.documentum.services.dam.df.IStoryboardService folder, there will be an impl jar, in this case dam_servicesImpl.jar.
There you go!
Have you found the jar?