Diamond * city is where I seem to end up buying ammo. The vendors everywhere seem to 'recharge thier supplies and caps fast.. I built some Trader shops in my settlements and the higher rank (cost of caps) seem to regen lots of items and caps as well.
Mainly I spend lots of time even selling pipe pistols and raider armor (for example) to drain my "clothes" merchant whom happens to have more caps then say my weapons merchant.. Then buy what bit they have back.
Before I hit diamond city.. There is a trader at some near by diner (dont remember name) and it provided me with a better assortment.
(edit) While bartering selling loot to buy ammo, changing clothes to get chrasima up just for the extra cost is also a big thing, every cap counts. Just becareful when you head off on an adventure again.. I hate ending up at a shootout wearing a tuxedo (or whatever) and some sunglasses as the only denfense.
Also as my brother is chatting at me about this now I should mention we swaped our recievers out and so keep a balance of one gun type per ammo type has helped us. This way we dont find our sniper rife run dry when we get out of a close range combat situation with a pack of ghouls
* Diamond city has a 24 hour merchant at the general supplies next to the weapon dealer (also has a good stock of ammo types)
If on Pc you may want to try the Craftable Ammunition Mod: http://www.nexusmods.com/fallout4/mods/1018/?
I took the scrounger perk level 1 and I seem to be finding plenty of ammunition now. I usually sell the .38 ammo I find to buy other stuff.
Diamond City, Good Neighbor like others have said, and this perk (also lockpick). Near the end I had more .45 ammo than I can deal with, although I did use a lot of .308 (due to the awesomeness that is the Overseer Protector), I didn't dip below 300 after mid-game.
Another tip if you're into settlement creation is to create shops; I know at least weapons and armor shops sell ammo.
I usually just stock up from vendors. Sell them the ammo types you never use too. I consider my .38 rounds as extra caps lol.
Theres a few things I never sell.. Ammo, stimpacks, radaway.
Everything else can go for a price. All the vendors you can build with exception of medic sell bits of ammo. You can sell them anything it seems. So clear them out of caps then just buy what they have in ammo you need. Then go hit up Diamond city or your other near by ammo connection and inest your profit on what you run low on. Having one gun is also not a good way to roll out in the wasteland. A shotgun for indoor encounters or bugs. A sniper rifle to get a jump on your foes and end a situation before it even starts, and a close encounter semi rapid fire high impact dammage. I like having a pistol to hip fire as well against such vermin as mole rats, ghouls which are in my view best fought at a retreating battle when you hit swarms. Also here is a big one... Why waste a ton of ammo spraying down a bloat fly when hitting it with a trusty bludgeon costs you no ammo at all..
oh and grenades are fun too.
Of course maybe you are into roleplaying the peaceful wasteland diplomat.. however ghouls are hard to negotiate with.. Sorry Feral ghouls I mean..
Really you just need to get used to spending time playing the barter by selling loot and buying suplies but trying to stay on an earning paygrade for funding your adventures. Charsima is key in getting a cut on price and changing clothes is a good way or graqe mentats (but watch out as adictol is exspencive).
Gun nut is also a big one as modding guns to fire other ammo is a huge boost to useing up surplus. Scrounger might be good I cant say, although every bit helps.
As your level gets higher then vendors will carry more .44 ammo. I'm finding 20-50 rounds on average in a few vendors at level 26 when it used to be only a couple bullets at lower levels.
I can sell you some ammo. I dont use it anymore and I will trade it for some fusion cores. Its all melee + gatling + explosives for me.
- high luck stat
- bullet search perk
- alternating between different guns/bullets
- not using automatic guns but hard hitting single shot weapons
- using perks that improve damage
i have constantly >1000 bullets on most major ammo type without really looking for it or using any special perks...having high luck alone helps
ps: .308 isnt "useless" - you just need to find the right weapon... i have a custom made legenary .308 which bleeds enemies for 25 damage (no armor saves) per hit - the pistol can shoot super fast and has nearly 0 recoil... i can shoot 10 times at an enemy in 2 seconds and go back into hiding while he suffers around 450 damage and i can shoot about 10 times in vats getting at least 2-3 crits.... its my best weapon especially against tough and armored enemies like power armors due to the bleeding....and since its .308 i got bullet for days...
I have one weapon for every ammo type, save for .308 which seems pretty useless. IMHO, half the fun of the Fallout series is making due with what you scavenge, as opposed to constantly grinding until you make enough money to buy that ultra-weapon like in other RPGs.
Scrounger perk two levels, and scavenge, scavenge, scavenge