While many of the firearms, and grenades, rocket launchers, etc come from elsewhere a decent percentage do come from the USA.
However it is not primarily civilian sources, most of these are military weapons sold to the Mexican government, a percentage of which then get diverted to cartels.
The government is corrupt at many levels, and anything the government has cartels with billions of dollars and strong desire can acquire.
If you trace many of the full auto AR style weapons they capture they often trace back to US manufacturers and the US government, with the Mexican government purchase in the middle.
A lot of Mexico's domestic police forces are equipped with M16/M4/AR15 weapons purchased from the US, and anything the police have gets to the cartels.
The US government also assists the Mexican government with arms and other things to fight the cartels, a percentage of which then get diverted to cartels.
Certainly a small percentage of US civilian weapons do get used by the cartels, in fact I know they are particularly fond of US handguns even though they don't play any significant role in the fighting.
The one portion of the population in Mexico that may in fact depend on US civilian weapons to be well armed is much of the civilian population that is not connected with the cartels.
A part of the population both the cartels and the government wish to keep disarmed.
They get more and more brutal down there all the time.
Now they are skinning people they capture alive:
http://www.borderlandbeat.com/2011/04/terror-in-tepic-two-men-skinned-alive.html
http://www.borderlandbeat.com/2011/04/nightmare-in-nayarit-another-body-found.html
Good reason to always fight to the death. Torture is a poor way to influence conflict for that reason, it removes all motivation to surrender to a superior force even when defeat in the battle is certain, increasing casualties for the winning side in future battles.