In a lot of detective stories there’s often a shady character who can find out questionable pasts/ information about suspects for the detective. They have widespread connections with both upperclass and the underworld. Do these people actually exist? And how would a character get in touch with them?
I’m going to be blunt for a second, characters like this are cheating. It’s not a deus ex machina, but it is a cheap way to consolidate exposition onto a single character. You’ve identified one of the weaknesses for the character in your question; how did the investigator first encounter this character?
With armchair detectives, this role often gets filled with a semi-sympathetic police detective. In that context, this character makes sense: they have a background that would familiarize them underworld activities. For example: If there’s a power struggle between organized crime families, it stands to reason that a detective who works that field would have some insight.
Similarly, a police detective is far more likely to know about criminal activities in high society because even they didn’t investigate it personally, they’ve probably heard rumors, or know the detectives who were involved.
Flipping this around, it’s not that outlandish to suggest a seasoned detective would have contacts in the criminal underworld. It’s a more complex situation, because those contacts would have to weigh the information they’re giving the investigator against how much it would expose them to reprisal.
If the contact is criminal, they might have insight on events in high society that were covered up. This could be the result of police investigations, or it could be the result of corruption.
Bridging the criminal contact’s information over to high society requires a very specific kind of cynicism about the world. Your setting needs to have solid ties between the people in power, and the criminal underworld. It’s not that this is an unrealistic cynicism, as there are real world examples where this fits. It also meshes nicely with noir as a genre, as that kind of criminal corruption elegantly fits the genre’s themes.
So, the short answer is, the right person, with the right contacts, and the right background, could know what your character needs. That’s a lot of things that need to align.
It’s just as plausible that your investigator would need to pick up each of the pieces individually.
So, let’s step back from all of this and talk about the genre: Mysteries, and this includes the entire detective genre, are puzzles. You’re presented with many pieces of evidence and asked to assemble this into a coherent chain of events. Your detective’s investigation is the act of collecting that evidence for the audience. This includes examining physical evidence, and also interviewing witnesses. In the process of their investigation, evidence and witnesses will lead to more evidence and witnesses. This is how an investigation (and a puzzle) grows.
I called this omniscient information broker as cheating earlier. The problem isn’t the existence of a witness who can finally give the detective context to solve the mystery, it’s when that character is omniscient and doesn’t flow from the investigation. This is the cliche you’re questioning.
If your detective is questioning someone, they need to be connected to the investigation somehow. This can be pretty flexible; for example, your detective might question people who worked maintenance or housekeeping for the building where the event happened. Maybe they think one of the employees saw something (either on the day of, or before.) They may question one of the participants’ associates in an attempt to learn about what was happening in their life before the event. They’re probably not going to wander off and check with someone, “because they know a guy.”
If your witness is giving information to the detective, you need to consider what they know, and also what they’re willing to reveal. A witness can’t tell your investigator something they don’t know, and they’re not going to (intentionally) provide information that will harm them. A character who knows all, and will share, is the antithesis of the genre.
Getting at secrets is something your investigator should be working towards. Who they are will determine what access they have. A cop or ex-cop will have vastly different resources compared to someone who was a friend of the victim.
Could you have a character that fits the cliche? Yes. As with most cliches, there are ways to make it work. They became cliches because they were very useful, and now the suspension of disbelief has started to crumble. There’s still the potential for interesting material here as well. Particularly if the, “omniscient” character has their own agenda and can’t be fully trusted.
Do these people exist in the real world? Actually yes, but not in the form you’re thinking of. Most people do become repositories of weird information over time. The exact intersection of criminal activities and high society has certainly occurred in a few places, so for example, a crime reporter in post-war LA, or 1960s Vegas would certainly fit that specific combo. A political operative in 1930s Chicago? Same situation. (And, without checking, I suspect I just described multiple James Ellroy novels.)
Do you need them? Probably not. In building your mystery, you can pick your witnesses, and you probably don’t need this specific collection of information.
How do you find them? By following the investigation.