If I'm remembering correctly, there should be one of those boxes in the alley on the right side of the house harboring Mark II and his gang, next to the unsuspecting bird and trash container. The Commando is the easy one to get. The "Kazo TRG", on the other hand, is a bitch, as there may or may not be a container nearby, given the random location of the sniper.
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Ah I didn't know that. Well, at any rate all those guns are mostly just collectors items anyway, as you start with waaaay too little ammo for them, and no neaty transportable rifle case.
I found myself fielding the custom sniper more than any other rifle, as it came with a box free of charge! :D