You can find this magazine on the eastern side of Remanzo province, inside a small Unidad observation post. The accessory case is in the armoury in the middle of the observation post.
The approach to this is very straightforward. The observation post is only manned by one sniper and four soldiers, and I easily picked them off with my sniper rifle.
This accessory extends the magazine capacity of .338 chambered sniper rifles, such as the SR-1, from five rounds to 10 rounds. I usually recommend these high-cap mags and this is especially useful for these bolt-action rifles, as 5 rounds is often not enough in Wildlands.