You can find this in north-west Remanzo, at a motel. There are four narcos here, wandering around the parking lot. The accessory case is inside the motel reception building, on the desk.
The enemies wander around and move behind the buildings where you can take them out easily when they’re isolated.
This magazine is for the SPAS-12 only (found in La Cruz), extending its standard 5 round ammo capacity to 8 rounds. As usual, a small handling penalty is the only sacrifice you make so this is probably worth equipping if you use the SPAS-12.
If you don’t have the SPAS-12, watch that video to see where to find it and how I think you might want to use it.