You can pick this magazine up in south-west Villa Verde at the edge of a small cartel controlled village on the edge of the jungle. The accessory case is on a shelf, upstairs in a wooden house on stilts.
I entered the village and tried to work my way around the perimeter to get to the house from the rear with the least resistance. You could take a wide approach and get to the house directly from the jungle which would be easiest. Other than one sniper on the roof, there’s one other enemy patrolling the porch, blocking your access to the steps. You have to access the stairs from the outside, by going around the front of the building via the porch.
This accessory takes your standard 20 round magazine ammo up 50% to 30 rounds for 6×35 chambered SMGs, which are… the SR-635. And that’s it. So if you use this gun in your loadout and really like it, or if you’re a completer finisher like me, then this is almost certainly worth picking up.