I buy ammo every chance I get, and I have the Scavenger Perk maxed so I find heaps of ammo.
I'm also running the Craftable Ammunition mod (for PC
) that lets you build an Ammo Workbench and make your own.
It's not cheap coz you have to make components from scrap and then make ammo from components but it's basically modelled off the way it worked in the previous games...