Holy heck, I needed this earlier today
Rick Wagner’s Blog: How to find which .jar a class is in (easily).
Makes mention of JBoss’ tattletale utility.
The comments also mention the Java Class Finder plugin for Eclipse (I used the CTRL+SHIFT+T today personally which did the same job)
There is also LibraryFinder plugin for IntelliJ and classjarsearch command line tool to search a directory of Jars for a class.