You can pick this up in central Ocoro province, at a Via-B Gas Station with three buildings, just off a hilltop road. A “Sicario Leader” side mission is nearby. The accessory case is on a shelf inside the small building out the front.
It’s difficult to take out the half-dozen or so sicarios here without spooking the sicario leader and failing the side mission. So if you plan to complete the side mission, I strongly suggest doing that first. But this is an easy province, so if you go hot, you probably won’t have much resistance.
This accessory simply extends the magazine capacity of the 5.7 USG handgun from 20 rounds to 30 rounds. Because it’s semi-automatic and used only for close range stealthy kills, and because pistols reload so quickly, I’m in two minds as to whether this is a necessity. You’re not likely to use up 20 rounds of ammo, and this accessory does slightly decrease handling. But it’s a small trade-off, so I do equip this on my side-arm and the 5.7 USG is the best handgun in the game in my opinion.