I'm a frugal guy and I don't like to sell/buy ANYTHING, except the occasional drug or misc. item.

As many of you know, weapons in the game either shoot in a semi-auto fashion or fire at 1000 rounds per minute, chewing through some of the easiest-to-find ammunition in the game. Other types are straight-up hard/impossible to find and a few (seemingly) can only be bought.

Killing people, even in the largest spawn areas such as forts, simply doesn't net enough ammunition. Not to mention, this rather barbaric method isn't particularly great for keeping a good reputation with a faction and only the most basic types of ammunition can be looted.

So, is there a better way? Picking an enemy's/ally's pocket maybe (if ammunition respawns, I really don't know)?

You either have to break down and buy ammo you need, or Farm it from re-spawning enemies.

Depending on the type...ammo is pretty cheap to buy, well worth spending the caps, IMO
I just buy every bit of 5.56mm, 10mm, .50mg, and .308mm that I can find. Oh...and pulse grenades, but that's about it. Weapons/ammo wise:)

I know you don't like to buy ammo but New Vegas has some good ammo deals. Some will give you a discount if you help them. FO3 I don't buy anything because there is nothing I need to buy that is not there on the ground.

Gun Runners have alot of ammo, can go to their store or to 188 where there is a Gun Runner. BoS base have a dealer
NCR has many.

But if you want to go the cheap way, looting big bases like fort mccarren. Killing Caravans might be a good place.

Reloading bench allows you to break down the ammo you don't need and use the powder and the lead for the ammo you do need. You just need the empty cases. Sometimes shops have empty cases you can just take at no charge.

Hand Loader perk allows you to recover more empty cases and unlocks reloading recipes.

Scrounger perk increases the amount of ammo you find in containers.

For those who use Energy Weapons it would be wise to make nice with the BoS, since if you do you get random free ammo every time you enter the bunker (it's in a crate to the left through the doorway at the bottom of the stairs). For Guns users, one possibility is to buy the ridiculous quantity of 12.7s Contreras sells and convert them to other types. If you're looking for more 45.70 Gov't rounds, Contreras and Gun Runners tend to have good quantities on hand every few days.

NOTE: Contreras' vendor chest is bugged and sometimes will not respawn, not sure if it's related to his unmarked quest or not since if you finish it in his favor his inventory changes.

The huge number of different ammo types needed to run a Guns character combined with it being harder to recycle the empties has me using EW instead, although my next character will be a Guns user now that I know how to keep them in ammo.

EDIT: after seeing how easily my EW characters can convert ammo back and forth while leveraging the BoS freebies I am tempted to mod the Guns ammo recipes accordingly. At the same time, my experiences with the Plasma Caster and YCS/186 have shown that doing so may massively overpower the AMR as well as anything that uses .45-70 Gov't, so a goodly bit of testing would be in order.

Scavenge a lot. You'll find the 'cheap ammo' and sometimes things like the .45-70 or .308s in random boxes and stuff. Alternatively, what you can do is scavenge tons of junk, and then trade them for ammo at Gun Runners or other vendors.

Enemy armor, weapons, and misc items can all be traded for ammo.