Well, you can go to places where someone can know. for example, she sells steroids, so maybe you can find a buyer ion the gym? And a missing person case was probably investigated by a detective agency
the wiki is lacking and there isn't a lot in game, but you can join the discord to ask people for help.
The gym I'll admit I didn't think of. But the fact that you get paid in drugs by the pharmacy if you don't have an ID, and then you sell those drugs in the Slums to get Silver made me think that would be the buyer. As for the Detective Agency, I tried going there and they just blew me off and said they didn't care.
