1. You can steal almost every single energy weapon there is from Silver Rush including Multiplas, Tri-beam, Plasma Caster, Recharger Rifle and Pistol, and Plasma Defender.
2. REPCONN HQ will net you unique Q-35 Plasma (comparable with La Longue Carbine in damage and very cheap in ammo spending) and Laser RCW as long as your Lockpick and Science permit it.
3. Gauss is in the Bear Bootlegging shack west from Sarsaparilla HQ.
4. Fiends around Vault 3 carry a ton of good replacement weapons so after you have Jury Rigging you can just go there every 3 days and gather what you need to repair your stuff.
5. Gatling you will have to buy from the BoS.

6. There are two normal laser rifles in Nipton. There's unique laser in Vault 22. A bit useless all of them - but enough to get by until you get Gauss.
7. Get 50 star caps, get Pew Pew, best energy pistol.
8. McCarran bounty target Cook-Cook will give you a Flamer (also Vault 22), Incinerator can be found in Primm hotel.
Here you have it, all energy weapons are easy to acquire. Ammo weights a lot but you can convert it between types freely.