That's a really serious one. I'll go looking for that story in Acts, Paul recorded it. Simon the Magus (the one famous for saying "who is Jesus Christ that I should worship him") also had this slave girl who the Bible describes as having "had a demon familiar" and he made a lot of money on her.
She can't have been a real prophet, because the Bible is specific in saying that she had a demon, but it also says that he made a profit, so I assume that she was accurate.