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
That depends on what you intend to hunt.
20 inch.............
Best bet is to search the internet auction sites such as eBay.com, GunBroker.com and AuctionArms.com sales@countrygunsmith.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.
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.
you will never go wrong if you buy the best you can afford