It can be found near Super Duper Mart on Emma's corpse. One can either go into mart and go down the stairs in the back, and go through the door labeled Commonwealth, or going around the outside of the building until finding an Advanced terminal to hack to open the garage.
Emma: What the hell is Josh doing? He's been gone for over an hour. We need to get out of here. The guys are thinking we're already at Concord. If Josh would hurry his ass up, maybe we can get there in time. Shit, gunfire. Not good. Josh!