You get illegal goods (cardamine, alien artifacts etc.) on bases in the fringe systems. I would name and shame the filthy rogues who peddle that stuff () but then this would be a spoiler...
You can also sometimes pick up illegal goods on the wrecks of smuggling ships. Not sure of the morals of that tho if you want to become a smuggler...

EDIT - (always forget the question..) You can make alot of money trading illegal goods but you can make nearly as much trading legal goods. Generally the smuggling routes take less time tho. Not much of a trader myself, possibly someone else has a better idea of the cash/time figures.

Edited by - redeye on 18-04-2003 01:40:41

The best trade routes I've seen all use legitimate goods. You can check the spoiler section for more details on them

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I'm sick of seeing all your signatures and not having one of my own