How long is the barrel on it now and is it threaded for chokes? I am using a 23 inch barrel for duck and goose with a modified choke. You may not need a barrel, but just a new choke.
However, many places carry new 870 barrels or you can get one on the EE. average is about 26-28 inches. I'd suggest one that has removable chokes. And get one with the same chamber size as the reciver.