It's important to note that Star Trek almost always displays life on a starship, with conforms to regulations, and shows an almost naive viewpoint on the Federation.
However, there are some notable exceptions:
Kirk and his human command crew were fond of Romulan Ale, which at the time was illegal in the Federation, and continued to be into Picard's time. Yet this didn't stop them obtaining it on several occasions. One could surmise from this, that there were other banned substances/items which could be acquired through less than legal channels.
It should also be noted, that the current human psyche is shaped by a world in which trade and money are a core part of society. In short, we cannot completely imagine a world in which money is no longer needed, as some trade between entities will always be required.
SOCIETY
The "utopian" society of the Star Trek university, is, at it's core, a socialist community, where the needs of citizens are met to an incredibly high standard by the government (particularly on Earth), and indeed, at least on paper, Marxist communism would have worked in Russia, but only if the vast majority believe in it, much like Communist China today. But even there, this is a movement of money amongst the populace.
In such a society, as Arthur C Clarke wrote, "there are always those who will be more equal than others", i.e. Those in power will, to aggrandise themselves, obtain items which cannot be procured by the general population, in order to increase perceived status within society. These people have such strong control over the law and other elements of government, that they can essentially do whatever they want, within some form of limits.
Picard, however, posits that Mankind has outgrown these impulses, and for the vast majority, this may well be true, but as has been observed, there are plenty of people in the universe who are unscrupulous, and by the standards of the time, social deviants. They tend to live on the fringes, in self imposed exile.
CURRENCY
It is also a misconception when they say there is no money in the 23rd century. It is true that there is no money, but it false to say that there is no currency. The federation runs on a resource based economy, with that resource being energy (due to being able to replicate most things easily, via an advanced energy based 3D printer, essentially). Energy is not free, and must be generated by releasing what is stored in matter, as a raw fuel. that matter is usually deuterium (easy enough to obtain), and anti-matter (ridiculously difficult to produce in large quantities). There will also be more complex goods that cannot be replicated easily, such as produce from other cultures, and it is these products that must be traded for, with some form of cross culture currency.
That currency seems to be gold-pressed latinum (probably some kind of gold/platinum alloy), though there would certainly be other hard to get materials that different cultures would prize.
ATTITUDES
All this aside, when one considers that in this post scarcity society, money is no longer required to by goods (at least, between Federation citizens), and with much manufacturing performed by robotics, and given that everyone's material needs, as mentioned before, provided to a very high standard, there would be no need for jobs, in the traditional sense, so there would be a great deal of freedom for people to do pretty much whatever they want.
In such an environment, many people would turn to hobbies, and turn them into lifelong passions. For example, Ben Sisko's father runs a restaurant. Not because he has to, but because he wants to. Many would become artists, musicians, philosophers, inventors. Many others, seeking a life of duty and meaning would join Starfleet, or the Federation government, and others seeking a challenge might go to the colonies, where life is harder, or become a trader, or leave the Federation altogether. Such a society is not just post-scarcity, but post-consumerism.
The point being that in such a society, what is classed as "luxury" is very likely to be vastly different from what we think of today. Luxury items of today, are items which are rare, exotic, expensive, or unique. In the 23rd century, expensive is a non issue for the most part, unless you want your own galaxy class starship, which most people would simply laugh at. That leaves, rare/exotic, and unique. Rare and exotic items can still be obtained, but by less than usual, or possibly illegal channels, and unique could simply be a handmade item, or a work of art in a museum. So, I would surmise that either they obtain it legally, through the use of contacts, and "favours", or illegally, via theft.
