How do criminals get guns?

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How do criminals get guns? Where does it start? 
   
The USA is by far home to the largest producers and manufacturers of guns in the world. So how do you think criminals get guns? 
   
There are so many quotes by the NRA about good guys and bad guys with guns, but does the NRA contribute to stoping the bad guys access, other than making more guns accessible to them? 
   
What are your thoughts? Try to go past just one level, where is this originating?