I don't know if you will be able to get it, but I think your only chance will be from the same Claptrap you got it from the first time. But even if you don't get it, you've got an excuse to play through as Roland!One minor rant, though. I accidentally sold one of my backpack upgrades, dammit, and didn't realized until after I'd left the game (and therefore couldn't buy it back from the shop). I just want to take a moment to say that having to remember to "open" them is...dumb. Now I'm stuck at 39 and have to hope I get one in the second playthrough (once I finish all the quests in the first one) so I can get the "42 inventory slots" achievement.
--Patrick
Although what you should really do is play through BL2 as Axton, because deploying the turret as a longbow nuke is FREAKING AWESOME!