Where did they buy the guns?
Not at the local gun shops, there are none.
They are readily available. The police often help people buy guns. Sportsmen also get shotguns and rifles and hunting in Mexico is a reality. There were gun shops when I lived there, but you could get them at large outlets such as the National pawn shops too.
I've got a buddy that lives in Managua, Nicaragua (he's an American ex-pat). He got worried for his family because of all the crime and went to the police station and bought an AK47. He said that they'll sell anything for Greenbacks.