You can find this in eastern Caimanes province, insides Caimanes base, a large cartel base. The accessory case is inside the armoury in the middle of the base.
I used a boat to approach via the river to the north of the base. You can enter the base from that side where a fallen tree has broken the wall. Being night time, some of the enemies were sleeping, which reduced the patrols and I only had to eliminate a few sicarios before reaching the armoury easily. While you’re here, you’ll also find the SR25 sniper rifle in this base.
I really like these large magazines, and I don’t really notice the difference in handling – but I think having more bullets available before having to reload is well worth getting this accessory if you use one of these assault rifles.