Take a good look around the cement factory's reloading point since there's a hidden spot in the western building through a hole in the east wall, definitely worth checking out first. The traders at rostok base are your best bet if you can't find it there eugene birch and ragman usually keep some in stock or wreck at yaniv station might have one if you're lucky. For upgrades, look for technician screw in rostok or technician harpy at yaniv station, they'll help you enhance it once you've got one. If you're after better mags, there's this interesting mission near lisovyi involving vitya shepherd that'll get you access to high-capacity ones, pretty essential for longer firefights. Random stashes near slag heap or chemical plant might have one too but that's more of a gamble, honestly better to stick with the reliable spots first.

You might want to check the traders before exploring the factory, saves you some time if they have it in stock right away. Just remember to bring enough money since these aren't exactly cheap and the upgrades will cost you extra.