Where do you think the black market guns come from? Pay attention to the footnotes. Also note that the data (which is used by the article too) is not about primary source. Meaning stolen guns could be included in the category of having been obtained from a friend too.

https://bjs.ojp.gov/content/pub/pdf/suficspi16.pdf

300k-600k are stolen every year.

"That’s enough firearms to provide a weapon for every instance of gun violence in the country each year – several times over."

https://www.theguardian.com/us-news/2016/sep/21/gun-theft-us...

There are numerous city level reports as well if one cares to Google them.