Iron Demon Ranch sells mods for assault rifles only.
It sells I and II with every non rep synergy.
It also has 2 specials for mods.
The Crater sells mods for pistols only, it's in the back where cass sits.
It sells I and II of every no rep synergy.
Also has 2 specials for mods.
Bug and chug has shotgun mods like the above two vendors.
Muir processing has sniper rifle mods like the above vendors.
Overpass has rocket launchers and detonators
Coit tower has BMGS and infectors.
Will update this thread as I travel around the map.
Now you can buy keycodes in bulk, however they cost a little scrip.
The mod horde cache is the same price as a tier 4 lockbox and gives you 1 random scope, 1 random stock, 1 random barrel and 1 random magazine.
Another update, rep vendor weapons do more damage compared to non rep weapons.