It is possible to executeBack Slash outside of V.A.T.S. (without the required Melee skill of 50 to perform the move) by initiating a power attack (hold attack button) while moving forward.
Assassin - The Caesar's Legion assassin sent to kill President Kimball carries a special version which is "non-playable". This prevents the weapon from normally being shown in inventory, e.g. when pickpocketing the assassin. The weapon is otherwise identical to the standard combat knife.
Cass - Rose of Sharon Cassidy carries a special "non-playable" version of the weapon, also otherwise identical to the standard version.
Combat knives are incredibly common across the Mojave. They are often randomly spawned in containers and bins, or can be found in almost all weapon stores.
They can be found on some of the Omertas in the Gomorrah Casino.
One in good condition can be found stuck in a blackjack table in the Vikki and Vance Casino. Go into the main room, take a right, into the small room, and it should be stabbed in the table (must be stolen).
There's one on a table in the headquarters building in Cottonwood Cove.
One can be found in the second floor of a ruined building in Boulder City beside a couple of skeletons. You will also find a service rifle and a Ranger hat in this building, each in very low condition.
Honest Hearts One can be located next to a toolbox, first aid box, and a skeleton. All of which is found inside a boat, underwater, at the Virgin River in Zion Canyon. The combat knife is in near perfect condition.