Marlin probably has one and most likely won't sell it to you.
Under the lever
Absolutely not!
The Gun Trader's Guide gives $185 for the Marlin AS 30-30 in NRA Good condition
If you don't have an owner's manual, find a gunsmith. You can try gun shows and want ads to find a manual as well as internet searches. Good luck.
Try gun shops, gun shows, on line auction sites, want ads, e-gunparts.com
You have one of the variations of Marlin 1895 - the G, GS, S, Cowboy, or Guide Gun. The value depends on which model you have and who you can find to buy it. The prices run from $400 to $700.
model 55 the original marlin goose gun
Depending on overall condition.Your Marlin goose gun will range in price from 55-200 dollars.
Places to try would include Marlin, gun shops, gun shows, e-gunparts.com
Did marlin make a goose gun with a thirty nine inch barrel
30th edition Gun Trader's Guide says 1959-65, and 60-69 for a model 57M
The Barrel has to be removed to replace it. Any good Gun Smith should be able to do it.