Your best bet is the farm (loot) New Haven for good items. To loot New Haven follow this process:
1. Open a game in New Haven
2. Open all 5 weapon chests in town (http://borderlands.wikia.com/wiki/New_Haven:_Weapon_Crate_Locations)
3. Sell off any unneeded weapons and check the vending machines for weapons.
4. Save & Quit to Main Menu
5. Do steps 1-4 over and over until you've found what you're looking for.