You can find this magazine in central Libertad province, at a small truck park off a main road. There are just four sicarios in your way. The accessory case is on a shelf outside, behind the trucks.
Equipping this does sacrifice some mobility, however I think it’s worthwhile compared to getting caught reloading at a crucial time during a fire fight. I’d recommend you pick this up if you like to run the M4A1 or the bullpup ARs.