Where can I obtain lots of ammo?
Solution 1:
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.
Solution 2:
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.
Solution 3:
I've made at least a dozen characters at this point, playing through the game multiple times on various difficulty settings with all kinds of character builds: semi-auto pistols, semi-auto rifles, melee weapons, no-VATS sniper, high-luck, low-luck, high-stealth, low-stealth.
You can find ammo in lots of places. Just loot, loot, loot. Some places to look that might not be obvious:
Flat orange tool boxes. These sometimes contain a bunch of ammo instead of crafting materials.
Super mutant gore bags. Mutants like meat and ammo. You will find both of these things in their bags made of meat.
Locked safes. Pretty much always ammo in these.
Feral ghoul corpses. Yes, that ghoul was carrying .50 caliber ammo. No, I don't know what he planned on doing with it.
The only time I've ever had any issue with running out of ammo is on my current playthrough, which is on Very Hard using Commando (automatic rifles and pistols). Automatic weapons burn through ammo, as each shot does significantly less damage, VATS fires bursts whether you need the extra shots or not, and one usually ends up firing shots outside of VATS, wasting ammo on misses.
Even then, though, it's easy enough to supplement my ammunition supply by buying some. Vendors with ammo are all over. Junk vendors (Carla, Myrna, Trudy, etc) usually have some, and of course weapons vendors (Arturo, KL-E-O) have plenty.
So, in my opinion, if you're having ammo problems, you're probably using too much, rather than not finding enough. You can conserve ammo in various ways:
Use semi-automatic weapons. Sounds like you're already doing this. These do much more damage per shot, so you have to fire less rounds. They also let you control your fire better, missing and wasting less shots.
Use VATS. If the enemy is at all close to you, you'll probably land more shots in VATS than out of it. Every critical shot hits, even through walls or at extreme range.
Use crits. Crits are auto-hit, and do tremendous damage. Use luck and luck perks to generate more crits.
Improve your weapons. More damage per shot means less shots total. More accuracy means less wasted shots. You don't need crafting perks to improve weapons. Pay attention to loot and weapons in shops for mods you can remove and attach to your own weapons.
Let your follower get kills. A follower with a good base weapon, like Danse, can fire effectively forever. Giving them a melee weapon works for followers with crappy base weapons, like Piper.
Use a melee weapon. This should be self-explanatory. I made a melee character on survival that can kill an on-level sentry bot with one sneak attack, or two to three non-sneak attacks. She only carries one gun (Kellog's pistol) and almost never uses it.
Use multiple weapons with different ammo requirements. If you're always finding .38 ammo, keep a weapon on you that uses it. You can kill mole rats and other stupid stuff with it, saving the good ammo, then switch to your real gun when you need it.
Use explosives. These do huge damage. Get Demolition Expert 2 so you can see a throwing arc for grenades. This lets you accurately place grenades among groups of enemies, so that you don't have to waste a bunch of ammo on all of them. You can fire 100 rounds into a mirelurk's shell, or save the ammo by dropping one frag mine and killing two of them.
Shoot weak spots. Many enemies have spots that are more vulnerable to damage than other spots. It's usually the head. Not always, though. Most robots have heads that are stronger than their torsos. You can determine which spot takes more damage by checking the size of the flashing damage segment on each limb in VATS.
If you waste less ammo, then the amount you find should be enough.