Everybody and their brother makes stuff for 870s, it seems. The problem more likely will be not buying a bunch of stuff that you don't really need. Midway handles a good deal of shotgun stuff, as do Brownells.
Everyone has different likes and dislikes for working shotguns. I usually wind up with a shortened stock fitted with a good recoil pad, rifle sights (the factory kind, that come on the barrel), a Sidesaddle, a white light source and provisions for a sling. Some folks want more, others less- which is why I say it's better in the long run to start with a plain gun and add what you decide you need as you learn the gun and develop your skillset.
fwiw,
lpl