Don't know of a specific location, but I got mine in a chest in jakobs cove at L35. The zombies drop a lot of good snipes, shotguns, and rocket launchers. Pay a visit to Dead Haven if you have the DLC.
Another method for farming (obtaining) legitimate items is by opening a game and opening all 5 chests in the town of New Haven as well as checking the store inventories. Once you've done this, sell off what you don't want, save & quit and do it over again. This can be done as many times as you'd like and can loot you some of the best weapons in the game. Other than weapons which spawn off specific enemies/missions all guns will spawn in the town of New Haven. This type of looting is best done after playthrough 2 when all guns dropped will be level 48. Here's a walkthrough:
1) Open a single player game and port to New Haven
2) Open all 5 chests in town
(All but #5 on this map http://borderlands.wikia.com/wiki/New_Haven:_Weapon_Crate_Locations)
3) Check the inventory of both stores (Zed & Marcus), Sell off items you don't want
4) Save & Quit
5) Repeat steps 1-4 until you're satisfied with your loot or tired.
I don't recommend going for chest 5 on that map since it takes more time than it's worth. Plus you'll probably have to fight enemies and as I said, it's just too much time. You should be able to make the run in a about 2 minutes once you're familiar with all 5 chests and the routes to get to them. Be sure than you've done all the claptrap missions in playthrough 1 and the DLC that way you have atleast 42 inventory slots. Once you've filled up your inventory with items, have a friend host a game that way you can place ones you wish to save on a mule.
A mule is a character which is created just to store items. You can only store 12 items on each character so I recommend only saving what you'll actually use. To create a mule just have a friend host a game, drop the items you wish to save, then quit out (be sure you friend stays in game to keep your items on the ground). Rejoin the game with a New Character and pick up all of those items.
As I said, I recommend saving only what you will use and in that case only storing Orange items. If you save items which you think friends might want you'll end up with a boat load of mules and a bunch of items sitting in them.