Where can I obtain lots of ammo?

The Scrounger perk provides a boost to your ammo finding capabilities. There are four levels to it each increasing the amount of ammunition you find. Scrounger is a third level LUCK perk.

There are also ammunition boxes throughout the world that usually contain lots of ammo but I find the best approach is to use one gun per ammo type and switch between them as you run out of ammunition rather than having multiple weapons that use the same ammo type.

I struggled with ammo in the beginning and found that once you don't scrap any and all things you find and instead trade with vendors, you will get much more ammo than by just relying on finding ammo in the field.

I do not have Scrounger, so maybe that would have made looting ammo more reliable.