They get their actual "Assault Rifles", selective fire, free with their loads of cocaine or from corrupt military and police.
They don't need to buy expensive semis from licensed dealers in the US and risk smuggling them across the border.
As pragmatic criminals I'm sure they get what they want from anywhere they can from anyone who will sell it. The CIA is always helpful with that.
grenades, RPGs, shoulder fired rockets, anti-aircraft guns, night vision equipment, body armor