I use two different tools to locate classes in JAR files:
http://isocra.com/2006/02/jarfinder/
http://www.jboss.org/tattletale
I find jarfinder easier to use when looking up a single class (though I heavily modified it to make it even better), while Tatttletale is great for corss-referencing all of the dependencies for a project (indispensable when converting a project ot Maven).