Try e-gunparts.com, Shotgun News, Gun List, local gun shop, gun shows.
Try contacting Marlin parts. Consumer Parts Orders: Consumer Service Dept: (800)544-8892
A 40" barrel produced for the Marlin model 120 12 Ga pump...I have one! Scott
The price range on your shotgun is between 165-200 dollars depending on the amount of finish being between 60%-90%.The Marlin model 120 magnum shotgun was made from 1971-1985.
In referrence to the question concerning the model 120 pump shot gun having 8 rounds, My husband recently bought a Marlin model 120 pump shotgun. He said that it does have 8 rounds.
The model 120 originally sold for $150. One that is in mint condition will bring about $275. However, if you also have the MXR 40" barrel, it alone is worth about $200.
50-300 USD, 12 gauge pump
120
marlin 12 gauge bolt action goose guns sell for about $200 in great condition
50-150 USD
50-160
50-200 USD
100-300 USD