There are two places in the contaminated sub-bay (Ts-19) where you can find crates that may contain it (of the ASH-12 style rifle) One is in the shipping containers the upper level to the right area, past the sub. Probably the easiest to get to. Another spot is when going thrugh the area left of the sub, you will spot a broken ladder - jump to it, climb up, then go around, and you will find another set of crates that can be searched.