As others have said, services are normally in one of the Library/Services folders (~/Library/Services, /Library/Services, and /System/Library/Services). If just looking there doesn't turn it up, another option is to spy on what files are being opened when you invoke the service, with something like:
sudo opensnoop -n WorkflowService
... and then running the service, and looking through the results to see if you can spot the relevant file. If it isn't an Automator-based service, you might have to leave off the
-n WorkflowService (although you'll have a lot more irrelevant files to sort through because you'll be seeing every file anything in the entire OS opens).