Found during Samara's recruitment mission on Illium. It is located in the room with a weapons locker, after taking the elevator to the mercenary base, and fighting through the first wave of Eclipse mercenaries.
The room also has a hallway that leads to Elnora's hiding place. If the first Scimitar is missed, a second one can be found in the room prior to the Gunship fight. A third M-27 Scimitar can be found after your encounter with Niftu Cal, just before the final encounter with Captain Wasea.
Mass Effect 3 Best Weapons
For Mass Effect 3 on the Xbox 360, a GameFAQs message board topic titled 'Single Player best weapons (DLC included)'.
Mass Effect 2 Guns
Mass Effect 3 Weapons
If the weapon is not picked up at any of these locations, it will be awarded at the end of the mission.M-300 Claymore.