go to eBay lol
Check under the forearm
Take the forearm off, it's usually on the bottom of the barrels.
J Janssen was one of the partners of Simonis, Janssen and Dumoulin which was a Belgian gunmaking firm that produced shotguns for the American hardware store trade from the late 1880's through about 1910. Value is for decoration, and rarely exceeds $250.
You could check midwayusa.com. If they don't have what you're looking for, I would suggest going to a local gunsmith.
To find hammers for old rabbit ear shotguns check with Dixie Gun Works
Remington cowboy model or Itt international both are stage coach style guns that go for about $300. Besides that there are side-by-sides offered from most of the major gun manufactures but most go for about 2k.
Reccomend looking at the parts listings at Gunbroker.com. Also periodicals such as "The Gun List" and Shotgun News. The Liege model is not often found, and a forearm can be difficult. Its possible that an early Citori forearm could be fitted and something to check out. Also try the Gun Parts Corporation on line.
If it is a Side by Side like my Western Long Range Gun, pop off the forearm - mine is marked in three places: the wood of the forearm, the metal of the forearm, and on the action. My Model 37 has the serial number stamped on the front of the reciever, opposite the slide for the pump. It is in the area between the magazine tube and the barrell.
One of the best sites to find information about the forearm is Wikipedia. Wikipedia will give you the best and most information about the forearm. Orthopedics.about is also a great site to check out.
Since the company has been out of business for 97 years, you'll probably have to find a gunsmith who can custom make it or modify a forearm from a similar gun.
In the forearm.