Too many wizard thieves would sooner or later draw attention to what they were doing and someone would make it their business to make them stop.
I'm not suggesting a wizard be a career thief. Just burgle a million dollars or so from criminals. After that, one should be able to invest that money in income producing property and live well enough off of that, especially if the wizard does the occasional "wizard" job.
Other ways to make money:
Treasure Hunting: I think someone mentioned finding and recovering shipwrecks. But we don't need to limit it to shipwrecks. Any sort of ancient sites or treasures could be found by wizards when scientists couldn't otherwise. Even locating archaeological/paleontological sites that don't have any "treasure" would be a valuable service someone would be willing to pay for.
Bounty Hunting/Informing: Simple bail jumpers to those $25 million bounties the U.S. government places on terrorists. This could obviously get pretty dangerous, but the great thing about those $25 million (and a lot of crime informing) bounties is that you only have to provide information. Depending on your sources, this could be easy and safe or dangerous and hard.
My brother has proposed making or recruiting spirits of intellect and selling them to wizards so they (the recruited spirits and the wizards) could have access to the internet. The "making" them is based on Evil Bob's existence. I'm not so sure that's an easy or feasible thing to do, but I'm also not sure it isn't. The recruiting is based on enticing them with access to "all human knowledge," i.e., the internet.
Cheating at casino's is something suggested, indirectly, by the text. There's a line in one of the descriptions of Mac's about a practitioner who has "enough kinetomancy to" cheat at dice. With $139, one could place three bets on a roulette wheel and make about $400,000. See
https://en.wikipedia.org/wiki/Roulette#Full_completes/maximums. Betting $139 on a single 35 to 1 bet yields $5,004. A second 35 to 1 bet of $1,000 (the table maximum according to some gambling website) yields another $35,000. A $40,000 "complete bet" yields 392,000. I imagine their are other ludicrous bets to be made with similar results on other games of chance. There are several accounts of people using math and engineering to legally take casinos for over at least million (either dollars, pounds, or euros), and that's just on the roulette page I linked to. I don't know how long, and for how much, you could gamble before getting into one kind of trouble or another, but you could definitely make enough to live comfortably, if not extravagantly. I'm not sure if this would work in the DF because Vegas is a very supernatural city according to the Paranet Papers.
If you could make somewhat accurate predictions of the future that are only occasionally perfectly accurate, you could make playing the lottery a no lose situation and would eventually hit the jackpot.