Engineer's Trading Post
The Engineer's Trading Post (ETP for short) is a so-called "secret community" of Starfleet engineers and operations officers. Initially founded by a young Ferengi ensign way back when, it's told to be a contact book of people officers and engineers alike can trade ship parts with. Though this activity does happen, it is more likely a ship part will be traded with a thank you of sorts. Ultimately, the end result is that both parties are happy - one with a part they desperately needed, the other with a story to tell, most likely with holographic evidence.
Perhaps it was that reason why many tried to keep this "community" off the radars of officials and captains.