It's not possible to tell where the Tesla rod is, that's Random. I think it's easier to find one in reactor deck, has a bit higher chance (at least my experience)
I mean, on wiki page it says that there is only two boxes that spawn weapons, yes it is random and not guaranteed, but knowing them would drastically reduce grinding time.
Well, nevermind, I managed to complete my quest in reactor and then on upper deck.
And while I was getting to the stairs up I finally got my Tesla emitter in the box behind the barrier
. So it seems that this is made due to balance reasons so that mage wouldn't get that weapon for free in the very first chamber, they first need to get the green card.
And wtf, I've beaten the game on hard with evil ending and all I've got was this lousy achievement
and not even the achievement that I've completed the game on hard!
Now I should repeat everything from the very save on dragon beach before sir valeran...