Depends entirely on what your looking for. Personally I think your better off grabbing a 700 ADL for a few bucks more, the 770 is a lower cost rifle and you get what you pay for. Not nearly a tight and smooth and can rattle a bit ;)
Remington, gun shop, e-gunparts.com
Wal-mart
You can buy a Remington brand of riffle since it is better than most of the other riffles.
Website for gunpartscorp
I can sell you my 591 only thing is I don't have the mag if interested my email is garyschwegmann1979@att.net
Almost every gun shop worth its salt should have a slug barrel for the remington model 870,if not then they should be able to order one with no problem.
Best bet is to search the internet auction sites such as eBay.com, GunBroker.com and AuctionArms.com sales@countrygunsmith.net
That depends on what you intend to hunt.
20 inch.............
Where Can I Buy A Clip For A1966Enfield Rifle Where Can I Buy A Clip For A1966Enfield Rifle
Check with a gunsmith. My usual source of parts for obsolete rifles is sold out of stocks for a Model 34, but a smith may be able to locate one for you. That is a very old rifle, and new stocks are not available- and used ones are scarce.
1816 - Legend has it that a young Eliphalet Remington II believed he could build a better gun than he could buy, and he set out to craft such a gun on his father's forge located at Ilion Gulch, New York. In the autumn of 1816, Eliphalet entered a shooting match with his new flintlock rifle, and while he only finished second, his handmade rifle was a success. Orders for new Remington-made rifles and barrels began to accumulate from the many other contestants, and before Eliphalet left the shooting field, he was in the gun business.