It's a random drop, like nearly every other damn weapon in the game. Look at the 'Guaranteed Drops' page and check the page of the weapons you're seeking to see if they're a quest reward or not. If not a quest reward or a guaranteed drop, they're always random.
I vendored two today, I have 2 more in my bank, I saw one for sale today at a vending machine that I didn't buy, I use one, and I vendored an old one a while back. Unless I'm the odd man out on Hellfire drops, you'll get one eventually. I found most of them in red chests. Since the level scales, you can find one in just about any red chest in the game.
They are present, but at higher levels they appear less, and at lower levels they still appear less than a volcano or a rebel. But my first time though the game, I didnt even know it existed, but now ive seen 12 of them. Also keep your eyes out for a purple machinegun with a 4x fire multiplier, as that could be a almost as powerful gun that is called the S&S Draco (but will appear as a purple rarity machine gun). Good luck.
it shouldn't take too long to get a maliwan hellfire, the armory in the 3rd dlc has a good chance of dropping one or more. some of the hidden red chests in the original zones have a chance of dropping em better when higher level. I never have a problem finding a hellfire i have about 13 or 14 in my inventory a some in the bank, varying between levels.
From another contributor:
I had one drop from a skag I killed the other day, to my surprise. I have found them in Gen. Knox's Armory DLC, in addition to vending machines.
From yet ANOTHER contributor:
Search absolutely everything. Even though it's extremely rare, guns like this CAN drop from skag/trash piles, dumpsters, and anything else that can drop any kind of weapon. the Hellfire is an SMG though, so it won't appear in lockers.
From some unknown guy:
What the first guy said is true, you dont have to be in Knoxx and its not only for that dlc. If you're lucky, you'll get one from killing a rakk or midget or pretty much anything. So kill pretty much EVERYTHING (even the most tiny enemies), and search ALL chests. Because there is a chance it drops from ALL of them. One time i got it from a white chest. A white chest. And white chests are known for having the worst guns and stuff but i was so lucky i got a Combustion hellfire (the only prefix for hellfire, you get it if you're lucky).