I assume you mean the Mark One trench knife?
These are improperly called the 1918 because that date is cast in the bronze (not brass) handle.
The true 1917/1918 "knife" is the triangular "ice pick" bladed knife with the walnut handle and stamped steel knuckle bow.
The Mark One is now a high dollar collectible, especially if it comes with the lron scabbard with both belt prongs still in place.
Other then big gun shows the only place I know of that almost always has one or two is Great War Militaria.
They usually have a couple of 1917/1918 knives and the Mark One. Prices are eye watering.
https://www.greatwar.com/scripts/default.asp
Here's one they have right now:
http://www.greatwar.com/scripts/prodView.asp?idproduct=2689055
You can also often find them on Gunbroker.com under a search for "trench knife".
Fortunately, the "replica" Mark One knives are so poor quality it's very easy to ID one of these modern Chinese made pieces of trash.
Here's a real Mark One:
Here's the current Chinese junk that's not worth having. The "scabbard" is made of thin cheesy steel barely crimped together. It's common to open the box and find the scabbard has fallen apart. The prongs can be pulled off with your fingers, and in no way is it usable as an actual scabbard.
A short history of the US Mark One trench knife.........
When the US entered WWI we had no general issue combat knife so Henry Disston & Sons designed a trench combat "knife" adopted as the 1917.
This had a 9 1/2 inch long triangular shaped ice pick-like stabbing blade, a walnut handle with finger grooves and a stamped steel knuckle bow. The nut that secured the blade in the handle could be used as a "skull cracker".
The scabbard was made of tubular green leather with steel throat and chape on the bottom. It had a standard wire belt hanger hook and was made by the Jewel Belt Co.
The knife was modified in 1918 with a different knuckle bow. The first type had pyramid projections on the center of the knuckle bow, the 1918 had the sides of the bow formed and bent into a double row of teeth.
Since both types were marked with 1917 and 1918 dates calling one the 1917 and the other the 1918 really isn't correct.
The 1917/18 was a failure and was unpopular. The throat of the scabbard was made so that it could only be used by the right hand, and the large open knuckle bow and long knife made it too easy to get caught on other gear.
To be an effective fighting knife, a knife has to have a sharp cutting edge that will allow slashing cuts.
Since it had no sharp edge it could not be used as a real knife, so the soldier had to carry the 1917/18, a bayonet, and still needed an actual knife to use for general purposes.
As a result, Major Eugene McNary of the AEF designed and patented the Mark One knife.
This was an actual knife with a double edged dagger blade that was an almost exact copy of the French trench knife blade.
The handle was made of cast bronze, not brass, and had sharp spikes on the knuckles. The actual purpose of the spikes was to prevent an opponent from grabbing the knife hand and increased the damage done with a punch.
The cone shaped nut that holds the blade in the handle is blackened steel and was usable as a skull cracker.
The blade was finished with dull gun bluing, and the bronze handle was given a chemical black finish. This black finish is usually worn off, or even polished off knives found today.
The Mark One was made by Landers, Frary & Clark, Henry Disston & Sons, and in apparently a very few experimentals made by Onida Limited.
By far, most Mark Ones were made by Landers, Frary & Clark.
The makers initials are cast in the side of the handle along with the large 1918 date, which causes people to improperly call the Mark One the "1918".
The iron scabbard was made by Landers, Frary & Clark.
It's made of two pieces of stamped iron, a front and back half, crimped together.
There are double lines of slits on the top and bottom, front and back to act as springs to keep the knife from rattling and to help retain it in the scabbard.
There are also pressed in retention "dents" on the top and bottom to prevent rattling.
The scabbard was copper plated then chemically blackened to prevent rusting.
On the FRONT of the scabbard are two belt prongs. These are riveted on the scabbard with copper rivets.
The upper prong is bent upward. (Many people think it's accidentally been bent this way).
The prongs were intended to be hooked into the eyelets of the cartridge belt with the upper prong inserted up into the top eyelet.
It was the prongs that were the weak point of the scabbard.
Since the Mark One was too late for much use in WWI, most were issued to Rangers and Paratroopers in WWII.
Not knowing that the scabbard was intended to be hooked INSIDE a belt, soldiers hooked them on the front of a belt and a good tug would snap the prongs off.
Also, many paratroopers didn't use the cartridge belt, preferring a pistol belt. In order to mount the scabbard on a pistol belt they'd bend the upper prong down and it would snap off.
For this reason, most scabbards are missing one or both prongs.
Because the prongs got broken off, it's common to find the Mark One with all sorts of expedient sheaths made from leather, canvas, or modified M1 bayonet scabbards.
Often a leather or canvas sheath will have the iron scabbard inside, missing both prongs.
Because the Mark One used critical bronze for the handle and the design was not really suitable for a combat knife in WWII type combat, when the supply was depleted the US M3 combat knife was developed and issued.
Still, the Mark One was very popular with paratroopers throughout the war, when they could get them.
Since it would take some time to get the Mark One produced in America during WWI, the AEF contracted with French companies to produce a version of it in France.
The blade was unfinished steel stamped with a reclining lion and the name "Au Lion".
The handle of the French version was roughly cast brass instead of the American bronze handle knives.
The handle was much cruder in shape and lacked the pronounced spikes on the knuckles.
There were two versions of the Au Lion, one with long grooves along the top edges of the handle to give a better grip.
The scabbards were blackened steel European bayonet type scabbards with the two prongs welded on.
During WWII it was a common practice to cut one or both sides of the round guard section off the handle.
This allowed the knife to lay flatter against the side and helped prevent getting the knife hung up on equipment, especially during a jump.
This was also often done to allow the knife to be used with a standard leather knife sheath after the prongs were broken off the iron scabbard.
After WWI the unsuitable 1917/18 trench knife was declared surplus and all were sold off as surplus. Although it's at least possible a few might have been found in some warehouse corner and issued during WWII, most of those carried in WWII were the surplussed knives picked up by a soldier or knives a father brought home in 1919.
Some Mark One knives were also sold as surplus after WWI, and these can often be found stamped with advertising from outdoor supply houses as gag gifts to good customers, often as "Bear defense" weapons.
After WWII, the Mark One was sold off as surplus in outdoor magazines, usually without the scabbard for as little as $9.95 in the 1960's.
Here's how the Mark One was intended to be carried: