You should get this this while saving all survivors. While going for that achievement, you will have to give up your Zombrex to another survivor. To get another, you must buy one from the pawnshop for $25,000. This may sound like a lot, but if you rescue Gemini and then do her sidequest (after you save her, leave safehouse and you'll hear her crying). The item she wants you to find is on the right side of the road between two white vans leading to the Quarantine Camp. You will have $30,000 given to you total for saving and helping her. Now you have enough to buy the extra Zombrex. Go to the store and purchase it, but remember, you must pick it up from the ground for it to go into your inventory.