The suppressor is described as:

Reduces the sound of gunfire at the cost of stopping power.

This is found in south-eastern Inca Camina province, near the corner of the map in Nuevo Mondo village. The accessory case is on a shelf in a building in the middle of the village.

There’s nothing else nearby; No side missions and no enemies, so get to the village however you like – I parachuted in from a distance – and just take the accessory. You’ll want to enter the building via the door next to the burning barrel and you’ll also probably want a helicopter to exfil with because it’s so remote, so probably bring one with you if you can’t summon one.

The choice of shotguns in the game is fairly slim pickings, and of the four, only the SASG-12 (which you can pick up in Agua Verde) can be suppressed.

In my opinion, it’s well worth it. You sacrifice a little in the handling department but the added stealth is just another reason the SASG-12 is my favourite shotgun in CQB, and ideal for clearing out those network relay stations.

The description says you lose some stopping power but the stats don’t bear that out, and with the SASG-12 being full auto, it doesn’t matter anyway. You may have to go totally out of your way to collect this, but give it a try. I think it’s awesome!