I Use Them For Smuggling

This isn't just for transporting contraband - you have to be scanned and have the contraband not be detected.

This needs a ship with a shielded cargo hold. Some vendors sell one (e.g. at the Den) - but a ship with one already fitted is the reward for the Mantis questline, which becomes available when you pick up a note about a Hidden Outpost from the body of a pirate you'll randomly kill during normal play.

Once you've got the shielded cargo hold, now for some contraband! There's randomly some contraband in various POIs and bandit camps - you'll inevitably come across some during normal play sooner or later. Failing that, you can steal from security checkpoints or jails - e.g. at Neon.

Note that only the first X units of cargo are shielded. So if (for example) your ship has 100 units of shielded cargo hold, only the first 100 units of stuff you put in will be shielded. You might need to sell/dump your other stuff first to make sure contraband gets those first few units. Or put it in your pocket and just be over-encumbered for a few minutes before you put it all back in again after putting the contraband in the hold.