This magazine is located in the middle of Koani province, inside a Unidad combat outpost. The accessory case is in the armoury toward the middle of the base on the north-west side.
You can approach this from the back of the base on the north-west perimeter to get in a small open entrance and sneak to the armoury in the middle of the outpost. Even at night, Unidad don’t sleep but there aren’t many soldiers for the size of the outpost, so you can sneak in.
First… this is actually the 20 round magazine, and this weapon accessory shows as the .45ACP extended magazine (20) in the gunsmith. I’ve reported this to Ubisoft, so we’ll see if it changes.
This extends the magazine capacity of .45 ACP handguns such as the P45T that you start the game with, from 12 or 15 rounds to 20 rounds and is well worth equipping if you use one of those handguns.