Enumerating for convenience...
- The White Council has vast financial resources and pays it staff a reasonable some. Given the hints about how much Harry was paid as a regional warden, this sum probably allows a normal but not extravagant life for anyone working for the council.
- Some wizard abilities related to information probably allow some wizards to invest in the stock market or otherwise financially benefit from information.
- Clearly some wizards are selling items of power to other wizards/supernatural agents and can probably make a good living at this. But multiple comments in the book imply that making items for other people is rather hard and requires a lot of skill.
- Some wizards are in the entertainment business (aka - old Morty) or otherwise "pretending" to be entertainers rather than real wizards. But few could make a real living in this way.
- Some wizards can probably have a "normal" profession where they excel due to (in part) their wizard abilities. And Ectomancer would probably make a good detective or a geomancer a good structural or mining engineer.
I expect #2 is much of what causes #1. I think it might even stray over into industrial / commercial espionage, on occasion, mysterious theft, or other "only if he can do magic" sorts of ill-gotten gains. Breaking mortal law may be "in bad taste" or something, but doesn't violate the Laws of Magic, so... no (wizardly) penalty. Other than wizards who find out, and may... snub you, or something? Speak disparagingly of how tawdry you behave?
The WC probably controls (or
IS) at least one major investment house in each major financial center in the world.
I think #3 gets many wizards some capital, which they invest via the WC "investment engine" ... and probably do very well indeed.
I think very few WC wizards are "in entertainment," per #4. Morty is a sub-wizard talent; poweful ectomancer, but that's just not enough... he has no non-ectomantic powers.
#5 seems possible, but honestly unlikely -- I think few wizards are willing to "work a normal job" like Harry is. But since you mention Geomancy... that'd be great for finding gold, or gems. Aquamancy might find wrecked ships, a few of which will have treasure, and many of which will have other value. Etc.
Mostly, I expect wizards do the #1 / #2 / #3 cycle: Work for the WC, or make items for the WC for "pay," and invest with the WC "financial wizards;" they're probably comfortably well-off after not very long... well-to-do shortly thereafter, and heading rapidly toward filthy rich, if they care to keep investing their time.
The WC is acting as their BANK. You know banks... they're a wealth-making engine! The WC probably does gloriously-well, as an institution, just handling investments for all their members.