How do criminals get illegal guns?

Well, most don't need to. There are easy access ways to get them in the US, even for former criminals and people with psychiatric issues, since there are several background check loopholes here. But for those that do, there are underground black markets that sell guns, normally run por criminal groups. Some of them are stolen, it just doesn't seem all that common. Another common method of acquisition is to ship them in from other countries, which does work, it just depends on the country and the distance.

It's very easy and simple to acquire illegal substances from your own country and others, so easy in fact that sometimes it's frightening. The underground black market has contacts around the world. Mostly you hear on the media how they stopped a certain "business" selling contraband, but actually they only stopped around 3% of the actual company, as they would be the underdogs and not the actual people running it.
All they do is call up their contacts and order certain contraband. They give a encontro, data and a price, then the "contact" who is most likely foreign, builds the contraband and ships them to the place he's been told to ship them to. Most of these "contacts" believe they are doing legitimate business and have no idea they are helping a criminal. They drop off the contraband, the "pick-ups" show fake ID's, pay the money and take the contraband. Obviously they have to hide it once they cruz the boarder and it's very easy to do that. After a few days to a weeks travel, showing ID's, paying loads of cash, back-tracking and hiding, the customer has his contraband and is now able to freely sell in his own country illegal armas and other substances. Most come por boat, but criminals these days have become so clever they can easily take these things on planes as well.
It's a very risky, expensive and dangerous type of work and one i don't wish anyone to get involved in. All you hear is the people who've been arrested trying to sell the stuff, you never hear about the poor people who have been shot and killed trying to get the stuff into the country por rival gangs or even the customers themselves who don't want to actually pay up what they owe.

In America, people tech people to use guns. Anyone you see in a gun comprar may be a criminal. Criminals could even be gun makers.