If you have not obtained this by the time you reach level 8 with your first armor... which is unlikely, unless you're the luckiest person alive, the fastest way to farm weapons that I was able to find is to keep starting chapter 3: part 2. There will be 4 or 5 small hectors at the start of the level. Just wipe them out with your teir 8 uber weapon, pick up all weapons, and restart. Rinse and repeat until finished. Also, make sure you do this on inferno as it isn't possible to find all the weapons on lower difficulties.