User Guide from CasaNova73099 from Survivalist General Discussions on SteamApart from buying them from some shanty towns traders (Jacob and Eliza West from Fort Kohai I know carry at least three between the two of them), you'll find one:
- Northeast from shelter (in the ravine) inside the trunk of an abandoned car, along with an Assault Rifle and some money. You'll find some Green Strains running around here.
- North (ish) from shelter in an abandoned car on the road to Santa Maddalena. Be careful though, since Red Strain is common here and if you're ill-prepared and they swarm you...
- Of course, there's the one you find in Dejado in the house across from the hardware store.
- You'll find one at the top of the mountain trail if you going on the "running date mission" with Alice. It's either in the convenience store at the top of the mountain or in the nearby abandoned car.
I'm sure there's plenty others that I haven't mentioned here, and of course, if you're the vicious type, you could always attack the traders of the shanty towns and loot them for their stuff. I usually never do this since that immediately places the shanty town the dead trader was from at war with me and I'm guaranteed to die from blood loss or a direct hit to the head, but hey. Sometimes you have to do what you have to do, right?