I'm going to disagree with the other answer.
I believe Mr. Norrell used magic to create his wealth. Norrell was a real magician, and a good one, at a time when there were at most a handful of other real magicians in England, none of whom publicly admitted to being able to do magic. Surely he could have figured out some way of creating or obtaining wealth. It's possible that Susannah Clarke hid some clues about exactly how he did it somewhere in the book (it's possible that she didn't), but if she did I missed them when I read it.
The other answer merely says that Drawlight claims that Mr. Norrell's wealth comes from a rich uncle, without making any judgment on whether Drawlight is correct or not. I think a closer reading of that section shows that it is quite likely that Drawlight has no idea what he is talking about, and is making things up to make himself seem knowledgeable. Later in the book, Drawlight turns out to be somewhat of a scoundrel, so this isn't at all incongruous with his personality. Here I analyze the scene in a little more detail:
Drawlight (the "small man") and Lascelles (the "tall man") are discussing the fact that Norrell has not turned up. Drawlight says
"I have said that I am disappointed," continued the small man, "but I am not at all surprized. You do not know him as I do."
Drawlight then remarks that Norrell has a rich uncle named Haythornthwaite from which he inherited the money. The author immediately casts doubt on the plausibility of this happening:
"Ha!" said the tall man drily. "He was in high luck. Rich old uncles who die are in shockingly short supply."
Norrell then comes over and introduces himself.
"I beg your pardon," he [Norrell] said.
"Yes?" said the small man sharply.
"I am Mr. Norrell."
The tall man and the small man gave Mr. Norrell two very broad stares.
So Drawlight, despite claiming to know Norrell, could not recognize him. It is later revealed that the reason that Drawlight thought he could recognize him is that he had been surreptitiously observing his house, and that he mistook Norrell's servant, Childermass, for Norrell. So Drawlight certainly greatly exaggerated his knowledge of Norrell, and it seems very likely that he does not know where Norrell got his money.