The complete way:
1) Lookup the bundle name: Feed "com.day.cq.wcm.workflow.process.ActivatePageProces" into http://localhost:4502/system/console/depfinder; this will show you the relevant POM statement for inclusion, but we're only interested in the bundle name. That's "cq-wcm-workflow" in this case.
2) Lookup the Bundle ID for this bundle. Goto http://localhost:4502/system/console/bundles and search for "cq-wcm-workflow" in the list. The number in front of the bundle name is the bundle ID. In my system it's 306, on your system it might differ.
3) Goto the console/ Windows Explorer and then navigate to crx-quickstart/launchpad/felix/; then go to the directory, which contains the bundle ID in it's name (in my case: "bundle306").
4) in a subdirectory (usually "version0.0") you'll find a file named "bundle.jar", which is the JAR file you are interested in. From there you can extract the class files.
Cheers,
Jörg