This can be found in eastern Villa Verde, in some ruins on a hilltop. The area is patrolled by five cartel gunmen. The accessory case is just inside one of the buildings, on the southern side of the ruins.
Despite this location being in the middle of nowhere and difficult to access, I don’t recommend approaching too close in a vehicle. It’s difficult to get to by land as there are no roads, only footpaths, and there’s not really anywhere to land a helicopter.
I drove up to about 120 metres away in a Zeus pickup and hoofed it along one of the footpaths on the eastern approach. I then simply used a flare gun to distract the enemies away from the accessory case and snuck into the building through a window for the pickup.
This magazine is only accepted by the M9 handgun (also in Villa Verde). Despite what it says, it actually increases the magazine capacity of the gun from 15 to 25 rounds, not 20, so it might be better than you thought, and worth getting if you use this gun.