Don't forget to revisit places like the toxic caves or Metzger, in the Den, when you gained some levels and found more equipment.

Learn the Location of random enemies on the map. Near New Reno for example, you'll find Mobsters and Yakuza. (maybe even fighting each other) If you indeed find them fighting, wait a little and then help the party that is losing. (most likely the Yakuza) Remember, the Mobsters only have 6 Shots. After killing all of them (Yakuza and Mobsters) take their Weapons. The Sniper Rifle is very good for any NPC and you of course, the knife of the Yakuza is great for Sulik.
Near San Francisco, Hubologist run around carrying HK G11Es. Just the thing for Sulik.
Near Navarro, you'll find Enclave troopers of course. One of them may carry the Gattling Laser (very nice)
Near the Military Base you'll encounter Deathclaws, Aliens and Mutants. Quite okay for early level (maybe 12 or 14) XP-gaining.
 

Normal and Special Traders

North of Modoc: Trader Wily has a random collection of items, sometimes, he has the extremely powerful Alien Blaster Pistol for sale. Make sure you find this guy.

South of San Fran.: Another trader, he sells the XL70E3

New Reno: If you become a made man, the weapon store has some high tech gear for you. Take the back door and go to the basement to find Algernon, who can upgrade Big Guns and Energy Weapons for free.

NCR: Buster, has three shelves of various items. Sometimes he offers 2mm EC or 4.7 mm Caseless ammo. His guards all carry Bozars, if you want to, steal one. All in all, quite a nice shop.

San Francisco: The two best stores in the game. The arms dealer sometimes even has a Gauss Rifle for sale. The general store also offers a fine selection of books and manuals. These guys each have two shelves of items for sale.