We know that Doctrix Minuette Colgate is a Dentist. She hands out free samples of Colgate.

Miss Lemon Hearts works at the Palace.

Miss Twinkleshine supposedly has a job.

Miss Lyra Heartstrings is a musician.

Miss Moon Dancer has no job (she spends all of her waking hours studying), but has an house and has not starved. I have an hypothesis about how she makes a living:

* She inherited the house, so did not need money for buying it.
* Her source of food is grazing in the park and soup-kitchens.
* She buys books only if the library does not have the title*.

* One can stretch book-money by reselling the books back to the bookseller. In my town, we have a used bookstore selling books for 1.00 U$D. One can sell used books to the bookstore for .50 U$D in cash or .75 U$D in storecredit (which one can only use in the store). If Moon Dancer only buys used books and resells them for storecredit, a small inheritance could last a long time before she can no longer buy books she cannot find in the library.

Ponies without jobs, if they can find shelter, can do much better than humans under similar conditions because ponies can graze in the public parks.

Moondancer might be independently wealthy, or at least living off a trust fund set up by wealthy (and indulgent) parents.

She is a Canterlot unicorn, after all, and we know that there are many of them who aren't hurting for money. Fancy Pants and the various wealthy ponies in his social circle, for instance...

Well Twinkleshine's cutie mark say to me that she could be either astrologyst or magican.

4540034 She is actually a choir member
Source http://mlp.wikia.com/wiki/Twinkleshine

4524924 Here are my views on their occupations.

Minuette naturally, is a dentist.

Twinkleshine is in a choir as indicated by the MLP wikia....a well paying one obviously.

Lemon Hearts works in the palace.

Lyra of course is a musician.

Moondancer is living off of her parents' generosity given that all she does is study. If her parents ultimately told her she had to get a job, she'd make a great librarian or even an author given her vast knowledge.