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.
The governor of Tabasco invited me to a bear hunt on his ranch.
I was not able to go because of work, he said he had plenty of guns.
I did not know at that time that they had black bears in Tabasco. I would have loved to have shot a jaguar.
I have never seen a gun shop or even an ad for guns in Mexico.