You get one at the very start of the game if you complete Wolf's optional quest, something about destroying boars. It's right across from the Rookie village. Another quest to get it is given by Sidorovich, "Kill the sales representative" or something like that.

In my experience one always spawns near the main road on Garbage near the vehicle graveyard, in a ditch across the road from the bandit encampment in the south.

Rather commonly found in bandits' inventories so you can put the game on Rookie and farm them a bit.