I just bought one from Chief Gustavo in Tenpenny Tower. It’s possible that the selections are scaled to your level, I suppose.
I picked up my first 2 from Bethesda. I had originally approached it from the outside, where after taking out about 5 or 6 raiders there, I had too much loot. So I fast travel back to Megaton to sell it and store it, and then fast travel back to Bethesda. Of course, knowing that fast travel will occasionally put you in a part of a location you haven’t cleared out yet, I was ready for a fight. I essentially was placed in the middle of six to eight raiders, two of whom had sniper rifles.
And later I went to minefield, where I picked up my third.
But those are the only one’s I’ve found so far.
And minefield also has a nice Chinese assault rifle in one of the homes. I find that most enemies use the standard assault rifles and Chinese assault rifles are usually in fixed loot locations. It’s gotten to the point that I’ve gotten so many assault rifles I’m selling them off, since right now I’m using the much better Chinese assault rifle anyway, so I have plenty for repair reserves should I ever switch back to nomral assault rifles.
I’ve also started selling off Hunting Rifles due to the fact that I use the sniper rifle for long range and everyone and his mother seems to be packing hunting rifles.
I’d save the hunting rifles, they are very useful for repairing the lincoln repeater.
Assault rifles and hunting rifles seem to be two of the most common weapons in the game. They were my cash cows, along with the raider armors.
I never had much of a problem finding the Chinese rifles in late game. Hint - if you travel to Arlington Cemetary and find Mama Dolce’s food plant, you can pick up a lot of rifles. I think when I cleared out Paradise Falls I got a lot too.
Wish I would have known that, I sold the Lincoln Repeater thinking that A: It was a relic and probably not that good and B: It would be hard to keep repaired.
I think all named unique items are better than their base version. You should keep them at least as souvenirs 
Interesting thing about the Lincoln, however, is that it uses .44 ammo and is repaired with hunting rifles. It’s a good idea to get Ol’ Painless from Dave in the Republic of Dave and the Lincoln, and just pick up the copious hunting rifles to repair each of them as you use 'em.
I’m finding myself hunting for super mutant brutes/masters to farm Chinese Assault Rifles, and hunting for Talon Company Mercs for laser rifles.
I save my plasma rifle for emergencies, Enclave troops being in somewhat short supply unless one’s actively hunting for them.
I’ve discovered a couple of things on my second round that I completely missed the first time around. First, always throw the wooden boxes around, especially if they’re on high shelves or stacked. I’ve found tons of skill books and even a hidden mini nuke that way. Second, always search the gore bags the super mutants leave around. You’ll usually get some caps or some ammo, and frequently .44 or .308.
Yeah, it always amuses me the way supermutants like to keep their ammo with their collection of mutilated legs.
Am getting quite tired of being a good character, though. Especially because every time I go back to Megaton, this idiot runs up to me and dribbles, “oh, thank you thank you for being so Jesusy, you’re my hero!” She then presses something into my hands, saying, “we scrimped and saved for weeks, and got you this.” I smile and nod, thanking her for the invaluable present. Ammo, perhaps? A stimpak, maybe? No, it’s instant mashed potato.
It’s just like Christmas at my aunt’s house. Am seriously considering shooting a random beggar so she’ll stop it.
Might do the same in Fallout. (honk!)
I am even nicer than that. I tell people “You need it better than I” Instant Karma (literally)
I always assumed the supermutant just threw in the bits of humans, and the humans happened to be carrying ammo.
How do you throw boxes around?
There’s a grab key/button, just grab something and swing it around to clear the shelves. I too have seen good items in a topless box under other mundane stuff. Bastards making me search every damn thing…
Knowing there is a grab key would have come in useful when Fawkes and dogmeat got wedged in a doorway.
Not sure if you can grab live creatures. You can drag bodies around, but the super mutants seem too heavy to move.
It’s “Z” under the default PC configuration. Kinda weird; it’s the very first key the tutorial teaches you (to pick up the ball or toys in your baby phase), but then it’s basically never needed again for the whole game, so you forget it exists.
As indeed I did. But it must be useful as a previous poster mentioned that you can grab boxes off shelves and find extra stuff in them, behind them, or under them.
On a seperate note: This is possibly the first computer game thread I’ve contributed a lot of posts to. It’s getting pretty big. What game holds the record for biggest thread, and how far off is this one? (I’m guessing a long way off)
edit: sorting by replies, the LOTR game thread has nearly three times the replies this one has. I guess this one has a long way to go 
second edit: That one doesn’t seem to be based on an actual computer game. this one seems to be top (currently) for computer games.
The main City of Heroes thread is currently at 17 pages.
And the various Diablo II threads were much longer than that one, IIRC.
But I don’t think you can really compare threads based on games with ongoing on-line content like D2 or MMORPGs with threads on a standalone, finite game like F3.
Interesting or humorous things I’ve seen.
Someone mentioned meaningful skeletons much earlier. Next to one of the radio towers there’s a car parked overlooking the wasteland. On the hood are two skeletons and an empty whiskey bottle. One of the skeletons is in the remains of a dress. I assume the other was male.
I watched a fight between a deathclaw and a Yau Guai. The deathclaw jumped up and smashed the yau guai into the ground. Literally into and through the ground. I watched the yau guai sink out of sight.
Dog Meat is my current companion. We were attacked by a feral ghoul and DM leaped at and was about halfway there when I engaged VATS with the combat shotgun. Single blast to the chest and bloody mess perk. The ghoul vaporized and DM sailed through the spot where it had been standing. He lands and turns to look at me as if to say ,“WTF was that all about?”
What can I say. Sometimes I’m easily amused.