I am planning on a story line in my game to include visiting an underground or black market. Is anyone aware of any pre-written modules or supplements that might be out there. I looked around and found surprisingly little.

Maybe take some inspiration from Hollywood.

There's a rumor that a seemingly legit business is a front for some sort of mafia people. If you go there and speak the code word (or do some other secret signal), you are taken to another location where shady deals are made.

There should definitely be a feeling that you don't want to cross these people.

I have in mind something like that. I don't have the time to write it though. I was looking for something pre-written in the form of an adventure of supplement that I could twist to my own needs. Since there is really nothing already out there I might have to write it.

Thanks for your thoughts.

Have you searched DMsguild.com? Im certainly not going to scour it myself, but I’m sure there must be something there.

if you can’t find anything you’re going to have to make it yourself, just don’t make it complex or detailed and let chaos take the reins.

- find business front

- learn the way in

- someone plants an item on a PC and accuses the “new blood” of stealing

- taken to leader to be “judged”, PCs think they are walking into their death sentence

- A twist, they get there “sooo, you’re thieves aye?” gives them quest to steal precious artifact from museum or personal collector

- blah blah etc...

If you are looking for a setting to slot in your market I'd recommend the Original Skullport setting description from way back in the day from Dragon Magazine. It's a wonderful and nasty place that would fit right in for a black market.  The article gives a lot of details on the layout and its principal businesses. (https://annarchive.com/files/Drmg172.pdf) and co-written by Ed Greenwood

That was the first thing I checked. Unfortunately and surprisingly there wasn't too much.