For fluff, the best option is of course: Read the Books (especially the Hobbit for Moria).
Yes, or FotRIf you want quick information, there are a number of good LOTR Wiki's:
http://lotr.wikia.com/wiki/Main_Page is the best IMO. Ordinary Wikipedia also has a lot of info. searching on youtube might help get a general impression.
lotr.wikia.com is my favorite as it has most everything. Avoid anything that has to do with the Battle for Middle-Earth games though.Short fluff on Moria: Once Khazad-Dum, the greatest and richest of the dwarf realms, the Balrog took it long ago, and Goblin's filled it and plundered it. It has huge hosts of troops (though these are rather useless), capable of surrounding the enemy and drowning him in soldiers. Taking it as Misty Mountains, it also gets hosts of Warg Riders (which GW strangely makes a point of ignoring), and one can add all sorts of evil creatures to the list. The most themed units would be: Moria Goblins, Moria Goblin Captains, Durburz, Warg Riders and Cave Trolls. Steer clear of Shamans of any sort, and to a lesser extent Marauders. Groblug is fine, so long as you get another model.
I would say that the Warg Marauders are GW versions of MM warg riders. Standard shamans fit into the fluff rather well, least ways as well as any of the non-named spell casters. The only ones that I would say aren't fluffy would be Druzhag and the Prowlers. Elite goblins don't make any sense. However, Blackshields make sense, because they represent the Uruks that Sauron sent to Moria that were mentioned by Boromir.Angmar: High in the north, the Witch-King has gathered all sorts of fell creatures: orcs, trolls and evil hillmen. All that unites them is fear of the Witch-King and hate of the Numenoreans. The ghosts which GW has added in WotR are not fluff, IMI (in my imagination, ie how I picture Angmar), however, not much is known about it, as it falls outside the great tales, so anything could be. Fluffy units: Orc Warriors, Morannon Orcs (to represent elite Orcs), Cave Trolls, Dunlendings, plain Ringwraiths. Avoid: Nothing that isn't obvious, really.
Not a fan of Morannons, the only Elite orcs that were mentioned in the books (or that even make sense) were Uruk-hai. Some Uruks would make sense in Angmar, as they were kinda everywhere.IMO, anything can be painted very, very nicely, so I'm not much of a help there.
However, keep in mind that this is very much my version of it, nothing is defintive.