The Viper Sniper Rifle can be found all throughout the mission you go to find Thane the Assassin, next to a terminal where Nassana is talking, next to the bridge you take to switch towers, and on a u-turn next to an elevator which brings down a mini-boss. Next the Canifex Hand Cannon is found in the mission you go to find Mordin, when you meet Mordin in his clinic he hands you the Hand Cannon. Garrus) when you meet him there is a bench in the room he is in it is sitting on it.

Later on in the game you can get Vindicator Battle Rifle, which is located in the mission you go to get Archangel (a.k.a. After you complete this mission you get the Avenger Assault Rifle, Katana Combat Shotgun, Mantis Sniper Rifle, or the Shurikan SMG in the opening of the next mission.

You first find your Predator Heavy Pistol in the locker at the beginning of the game, later in that same mission you come to an lift and beside it is the grenade launcher.

Every gun is spread throughout the game and can be found almost very easily, but you could be missing one or two cause you ran over them and didn't look.