All of the small wars underway in the world are kept alive by an informal, but international, network of smugglers. These outfits move guns to where there is a demand. All they need is buyers, with cash, and not a lot of cops or soldiers getting in the way.
Organized crime, that is, gangs that have been around for generations, are the core operators. Various Italian criminal organizations (usually referred to as "mafia") have been at it for over a century, moving goods and people across borders illegally. The gangs don't much care what they move, but what they get paid to move it. Currently, the most lucrative items are people (economic migrants, women enslaved as prostitutes, and terrorists), drugs and weapons. People are the most preferred, as you don't have to sell them after you move them. The migrants pay large fees to be moved from poor countries into Europe or North America. Drugs are more dangerous, involving some pretty nasty people at both ends.
Guns, like drugs, involve some risk, but if the money is right and the obstacles few, the deals will be done. At any given time, there are millions of rifles, pistols and RPGs on the market. We're talking illegal weapons. Most of the rifles are AK-47s (many models), SKS (sort of a semiautomatic AK-47 that looks like a hunting rifle). The pistols tend to be 9mm semi-automatics. RPGs are usually the 1960s vintage RPG-7. It may be old, but it gives you some artillery capability, and it's cheap.
Second hand AK-47s can be had for as little as $100 each, although delivery costs can more than double the price. If you want the lowest prices, you buy in bulk (a thousand or more at a time), and take delivery via ship. An SKS costs about half as much as an AK-47s, and most pistols cost less than an SKS.
Most of the weapons on the market are pre-owned, primarily Cold War surplus. The Soviet Union and its satellites manufactured over 50 million AK-47s, with most of them stored away for some major war, where these rifles could be handed out to hastily conscripted civilians and reservists. When the Cold War ended, gangsters, crooked politicians and weapons dealers were quick to put most of these weapons onto the market. There were so many of these weapons floating around, that, by the late 1990s, some could be had for under $50, in areas where there were just too many of them.
By now it's pretty obvious that, if these weapons are not destroyed, they will show up somewhere else before long, even if they are turned in at the end of some conflict. Thus it has become customary to destroy surplus weapons. Until quite recently, nations that upgraded their infantry weapons, would sell the old ones to an arms dealer. Big mistake, as many of these weapons, disposed of legitimately, would eventually end up in the hands of some bandit or terrorist.
There is one major source of new weapons, and that's the Chinese firm NORINCO. Owned by senior officers of the Chinese military, NORINCO is well connected inside the Chinese government, and not too picky about who it deals with. NORINCO is also discreet, and adept at making deals that are kept quiet. However, when various governments kill bandits and rebels, and find they were carrying NORINCO weapons, questions are raised. So far, NORINCO has denied everything, blaming unscrupulous middlemen. NORINCO manufactures and sells hundreds of thousands of rifles and pistols a year. Their version of the AK-47 and SKS are prized, by killers and collectors world wide, for their quality and durability.
In many parts of the world, these weapons are sold openly, or, at worst, "under the counter." Any area with a weak legal system, and corrupt cops, finds that this kind of trade thrives. If you have the cash, you get the guns. These dealers can, for a fee, arrange large sales, and it's through this worldwide network of small dealers that the major operators get leads for deals involving thousands of weapons, and millions of dollars.
In the wealthier countries, where the rule of law prevails, you have to work a little harder to connect with the international arms trade. But the connections are everywhere, and deals are ready to be made.