With the Remington 870 Express you already have an excellent, basic platform from which to build a tactical shotgun, as others have noted.
What are your needs? That should determine whether it's worth outfitting the current set up, or buying a separate piece altogether.
My guess is basic additions with a focus on home defense are the way to go, unless you're breaching doors and clearing rooms; I presume you're not.
So:
-- An 18" barrel with a bead sight or perhaps tritium dot (unless you're planning on putting lead on targets at outside-the-home distances, I don't see the need for rifle or ghost sites, and for me they get in the way at close quarters).
-- Synthetic stock with a reduced length of pull, and synthetic forend; shorter and lighter.
-- Side saddle for extra ammo.
-- Optional and debatable: magazine extension. I like them for the extra shots; some feel it unnecessarily adds weight and imbalances the gun.
-- Optional and debatable: a sling. To me they aren't needed for indoor work.
-- Optional and debatable: a light system. Cuts both ways; lights up the target and/or confirms a friendly, but also announces your position. Your call.
There's a place -- AI&P Tactical -- that deals only in tactical 870s. Haven't purchased from them but I intend to. I like the products, and really like the perspective of the armorer as it comes through on the site:
http://www.aiptactical.com/
One of the things he does to all the 870 Expresses is outfit them with upgraded internals from the 870 Police model; that's something you also might consider.
All this to say, I don't think you need to buy a new piece, just modify the one you've got.
But your original question was about manufacturers offering tactical shotguns right out of the box. Remington does have the 870 Express Tactical; not for me (ghost sites, choke) but might be for you.
Or a Mossberg 500, which is also an excellent platform, and offers a much greater variety of options from the manufacturer, increasing the chance of finding one that is suited to you.
If you're not interested in modifying, the Mossy 500 in one of its tactical configurations is the way I'd go.