I found out the smallest weapons shop does not carry .50-cal. Does tier 2 carry it? Does Weapons Emporium carry it?
This is my experience in fallout 4 vanilla.
Currently over level 80+. No scrounger perks. Local leader perk maxed, Cap collector perk maxed.
Certain type of guns and certain type of ammo cannot be found easily due to a level requirement in the game to get these items. For example: a level 40+ character will usually find more mid level weapons and ammo and easily find low level weapons and ammo while a level 25- will only find small amounts of mid level weapons and more 10mm, .38, .45 weapons and ammo.
I have some settlements with a weapon vendor. I scrapped the low level ones and now have tier 3 weapons vendors in sanctuary, red rocket, abernathy, and the castle. Most of these don't usually sell .50 cal ammo. But they sometimes have .50 cal ammo which according to my test have been at 6 (edit: was 20 but bought 6 just yesterday) to 200+ when buying them. It seems that the vendors in Diamond City and Goodneighbor are better due to them stocking it most of the time I check there.
It seems that according to my research in both the steam forums, reddit and the fallout 4 wiki that the scrounger perk at higher levels will enable you to find more .50 caliber ammo. As of now I currently have over 4000 of the .50 cal even without the scrounger perk due to me not using the .50 cal sniper rifles and relying mostly on my explosive .38 automatic pipe pistol.
My suggestion is to find a gun that works for you and you have ammunition for and stick with that as your main for a while. My build is having a weapon for long range sniping (a legendary two shots I bought somewhere), a weapon for short range (a legendary from a quest line that lowers VATS usage), a weapon for mobs (an explosive shotgun I found on a legendary deathclaw does the trick), a weapon for mid range (my main weapon an explosive .38 automatic pipe pistol that has been with me since level 15), and a melee weapon that causes staggering or stuns.
It is a heavy investment to get your shops in the settlement to a higher tier. Not just the materials needed but also the caps needed to do so. So I would suggest you look for your .50 caliber ammo in Diamond City or Goodneighbor and if you are lucky to meet up with the wandering weapons merchant they may also stock .50 caliber.
So to answer your question: Tier 3 weapons vendor does sell .50 caliber ammo. Tier 2 sometimes does too. But it is better for you to get leveled up and just search for a merchant and buy from them and also get scrounger perk for it to find it as loot if you really need the ammo.