Illicit, professional smuggler
As long as there are territorial governments in this world, there will be laws between them that differ and people who are willing to profit from the transfer of cargo from a place where something is available to a place where something is not readily available by law. These folks are known worldwide as smugglers. Weapons are generally regarded as medium bulk cargo and are not a favorite high profit item in industrialized nations. The main reasons for this are the durability of weapons and the profit to cargo volume ratio of guns versus more lucrative contraband such as narcotics. Firearms can easily have a useful life of a hundred years as long as ammunition is available. Once you import a gun and put it into an economy, it circulates for several years and just ads to the available supply. Profit margins diminish as the market becomes glutted. Narcotic supplies, however must me constantly replenished, thus ensuring profit for the career smuggler. Ten pounds of just about any narcotic will be worth more profit to a smuggler than your average assault rifle. More likely, you would be able to buy weapons smuggles in "spare room" of some other shipment or mixed in with bulk cargo. Another option would be to smuggle in key components of weapons while constructing the rest at the destination or only smuggle those key components which are otherwise unobtainable. An example would be receiver and action assemblies. Stocks and grip assemblies can be made in almost any basic hobby shop.