Fotu Ino the Marine Magnum comes stock with an extension built into the barrel retaining nut. It allows the Marine to hold 5 or 6 rounds in the mag. tube instead of the standard 4 rounds. You can buy magazine extensions from just about any good gun store, and spare barrels also. Mossberg is now making a couple of barrels designed to fit the Remington 870 and I have had good luck with them. They cost about 1/3 what Remington wants for a spare barrel. There are several different companies making magazine extension and other accesories.
Choate, Tac-Star, and Scatterguns.
I've used Tac-Star and scattergun mag. extensions and both are very good in quality.
I don't have any experiance with choate goods in actual use but they seem to be good in quality.