As Colombia has turned the tide on drug gangs and leftist rebels in the last decade, they have collected thousands of weapons used by the enemy. Serial numbers were checked, to find out where all these weapons (nearly 100,000 so far) came from. While 17 percent of the weapons came from the United States, these were mostly pistols, shotguns and rifles. The assault rifles (RPGs, mortar shells, grenades) mostly came from Eastern Europe. Out of all weapons captured, 18 percent came from Bulgaria and 13 percent from Russia. Almost all the weapons from these countries were assault rifles, mostly AK-47s.
The drug gangs and anti-leftist militias bought more of their weapons from the United States, but everyone got most of their arms from gunrunners based in Eastern Europe. That was largely a result of the end of the Cold War. The former communist countries found themselves with millions of AK-47s and light machine-guns (and many similar weapons) that they no longer needed. Communist countries were police states with very large armies and police forces. Most of these personnel were armed with AK-47s, although the majority of the troops were reservists. So their weapons spent most of the time locked up in armories. Since the 1990s, these armories were either looted (as in Albania and Iraq), or had their contents sold off, by corrupt officials, in illegal arms deals. China still manufactures a lot of AK-47s, and is willing to sell them to shady dealers, as are some manufacturers in places like Bulgaria. The AK-47s flooded Africa, Asia South American and the Middle East in the last fifteen years, making them very cheap (less than $100 in some cases).
The major gunrunners were known, but managed to find sanctuary in Eastern Europe and Russia. Another major source of such weapons are corrupt officials, who sell off weapons to anyone. Such corrupt officials also sell older weapons, instead of following orders and destroying them. The gunrunners found they could get some of the best prices in Colombia, where the cocaine trade provided more cash than found in their other markets.
Flash enough money, and they will come.