First of all, welcome in the forum, hope you'll enjoy your time.
1. Looting is the key, and your best source is just playing the game - doing quests, shooting hostile people (selling their stuff) and scavanging are essential. A high repair skill and the Jury Rigging perk comes handy, after this its guaranteed that you'll be swimming in the caps. Keep in mind that you can fix weapons / armors / etc. on your own even without focusing on the skill, but that'd take more raw material, therefore less profit. And the NPC's who has the ability to do that instead of you are expensive, paying for them is absolutely not profitable.
Other than that, once you made your way to the Strip, you can get decent amount of caps just by playing in casinos - with a Luck SPECIAL attribute 7 or higher it shouldn't be a problem. If you have caps, you can have everything. Well, almost, but hey, this is Last Vegas, right ... ?
As far as ammo, well ... you'll find a nice amount from more common ones, try to specialize (to max three different types, for example) and sell the other ones. This is a good way to get caps too. Not to mention that with the proper perk(s) and Survival skill, you can make ammunition too (which means you don't have to spend caps on it), even a few special ones. So the alternative to selling unnecessary ammo is breaking it down and use the acquired parts (powder, case, lead etc.) from it to craft the type you need. For further reading: http://fallout.wikia.com/wiki/Fallout:_New_Vegas_ammunition and http://fallout.wikia.com/wiki/Fallout:_New_Vegas_crafting.
2. From travelling merchants to regular ones, almost everywhere. Note that their inventory restocks two times a week (Wednesday and Sunday, if I remember right), so you need a little luck to find what you're looking for.
Also, if you have Gun Runners' Arsenal DLC then at their store, you can find almost everything. They are specialist, so its not a big surprise - hunting to weapon mods in general stores needs more patitente.
3. Just do their side missions, and finish general quests in their favour. More info http://fallout.wikia.com/wiki/Fallout:_New_Vegas_quests. (Beware with the spoilers). I can't emphasize the word
side before missions enough, this is what this game all about. Probably the easiest way is just go to their camps, talk to everyone and you'll end up a ton of things to do for them. After that it just matter of time and number of successful mission to reach the "Liked" status - by the way, the same goes for every faction, though. (Note that you can't be friends with everybody at the same times, you have to pick side time after time in certain parts of the game, which naturally causes to hurt other faction's interests.)