Well, basically, almost all the fluff is already in the rulebook, as well as in the Empire in Flames supplement for the surroundings. There were also some additional articles from the time the game was released, you might be able to still find them in the ressources section of this forum. Then there are some bits and pieces in the Town Cryers, too, I suppose.
The fact that Mordheim isn't mentioned in the current Warhammer is that Magnus the Pious completely destroyed what was left of the city after unifying the Empire again, as far as I remember.