I have found it in red chests, so far more often than any other "unique" weapon (6 times from levels 22-50 in multiplayer, and I don't go out of my way to farm specific chests either).
If you're looking for farm (collect) items here's a good method:
1 >> Open a Single Player Game in New Haven
2 >> Open all 5 chests in town and collect all the items.
3 >> Sell the items you don't want in the store and check the store for items
4 >> Save & Quit
5 >> Do steps 1-4 over and over until you've found all you want
Here's where the chests are: http://borderlands.wikia.com/wiki/New_Haven:_Weapon_Crate_Locations