I tried Gun Runners, but could not find a decent selection of energy weapons at the Vendertron. Where else could i find energy weapons?

The Van Graffs at the Silver Rush.

Yeah, The Gun Runners do not sell any energy weapon, as far as I know. They don't sell any energy ammo either.

Silver Rush in Freeside has a good selection of Energy Weapons that you can buy or steal. They have the Plasma Defender and Sprtel-Wood 9700, the GRA Unique Gatling Laser.

Cliff in Novac sells MF Hyperbreeder Alpha, the GRA Unique Recharger Pistol

The Fiends and The Brotherhood of Steel favor energy weapons. If you become friends with the Brotherhood, their store has pretty good selection of energy weapons, including The Gauss Rifle.

REPCONN Test Site and REPCONN Headquarters are also good sources of energy weapons and ammo.