I got it from a briefing by an LAPD guy teaching policing strategies to military in Iraq/Afghanistan (he was a reservist, but I think they did an official program later involving LAPD and possibly FBI and NYPD) -- i.e. how to shut down networks of supply for insurgents getting weapons.
One problem with saying "1.2% of gun dealers are responsible for most of the guns used in crime" is that a lot of dealers are very low volume or very specialty (target, etc.) anyway, so mainstream sales may happen mostly at 1.2% of dealers, too. I.e. most of the knives used in killings are probably bought at Wal-Mart, not the Global store in Tokyo.
http://www.pbs.org/wgbh/pages/frontline/shows/guns/procon/gu... is a decent source. There's another source I saw recently which talks about young or new guns being the majority of the problem, but I couldn't find it on google in 30 seconds.
One problem with saying "1.2% of gun dealers are responsible for most of the guns used in crime" is that a lot of dealers are very low volume or very specialty (target, etc.) anyway, so mainstream sales may happen mostly at 1.2% of dealers, too. I.e. most of the knives used in killings are probably bought at Wal-Mart, not the Global store in Tokyo.