Questing in Storm Peaks, you should stumble upon the Crown of the Hyldnir in the middle of the massive quest line started by They Took Our Men at K3. While 37 defense rating seems a little low, the helm does have a handy yellow gem slot, perfect for a Thick Autumn's Glow or a Thick Sun Crystal, bumping up that defense rating by a handy amount.
Of course, you may want to skip the hassle with the gem slot and just do some questing in Icecrown. By helping the Argent Crusade, you'll eventually get The Crusader's Resolution as a quest reward, a helm that not only beats out a gemmed Crown of the Hyldnir in defense rating and stamina, but also comes with some expertise, perfect for bumping up your threat.
If you're doing some dungeon running, keep an eye out for the Helm of the Ley-Guardian from normal Oculus. It not only beats out both of the above helmets on stamina and defense rating, it has a red gem slot, which might look good with, say, a Champion's Monarch Topaz in it. Really, if you have a chance to run the Oculus, just take it and pray for this helm to drop.
Whichever helm you choose, be sure to grab revered with the Argent Crusade and enchant it with the Arcanum of the Stalwart Protector to take care of a good part of your defense needs right there.
If you're looking for a non-dungeon option for your defense rating on the shoulder needs, consider finding a friendly Blacksmith to make you Tempered Saronite Shoulders. They're only level 76, but they have a nice bit of strength and stamina to go with their defense rating.
You might also want to keep an eye out for the Crusader's Square Pauldrons. They have the most armor of these three shoulder choices and some nice dodge rating that's just perfect for keeping Bone Shield up a little while longer. They drop off trash in the Culling of Stratholme, but are bind on equip, so you may find them on the AH as well.
If you have a chance to run the Halls of Stone, you'll get a very nice reward from a quest in there, the Pauldrons of Reconnaissance. They don't have quite as much stamina or armor as the other options, but they are likely to cost you less. Most importantly, they also have the most defense rating of our three choices by far.
Of course, once you get honored with the Sons of Hodir, you'll want to slap the Lesser Inscription of the Pinnacle on your shoulders as well.
Your best pre-heroic bet for this slot is probably going to be the Breastplate of the Solemn Council, from revered Wyrmrest Accord faction. Unfortunately, unless you've been doing all of the Wyrmrest Accord dailies every day for a few months, you probably aren't going to get revered with Wyrmrest without strapping on the tabard and running a few level 80 dungeons. Still, it's probably worth it. It has tons of every basic statistic a Death Knight tank needs, plus some expertise on the side.
While you're working on that reputation, you might consider picking up a Tempered Saronite Breastplate. Your local smith can make this for you, and it has tons of armor, stamina, strength, and defense rating that should serve you well until the dragons cough up your chest piece.
If you're not quite willing to plunk down the dough for the Tempered Saronite Breastplate, there's also the Cultbreaker's Chestguard, gotten from the Chain of Command quest from the Death Knight on your faction's Icecrown airship. Once again, a somewhat low 37 defense rating is offset by a yellow gem slot that offers the chance to make it up, plus an extra 4 defense rating from the socket bonus.
Finally, if you've been doing your Kalu'ak quests, you probably have access to the Ivory-Reinforced Chestguard, which compares very favorably to the Tempered Saronite Breastplate in defensive stats, and has some expertise besides.
As a side note, the chestal area is a good place to grab even more defense via Enchant Chest - Greater Defense, which will net you an extra 22 defense.
Tempered Saronite Bracers are a nice quick upgrade for level 80 tanking, with plenty of defense rating and a good amount of stamina. For "freer" options, you'll have to do some dungeon running.
Svala Sorrowgrave in Utgarde Pinnacle drops Svala's Blood Shackles. While they have less defense rating that the Tempered Saronite, they blow it away on strength, armor, and stamina, and have some dodge rating besides.
If you're running the Oculus, you may also want to pick up the quest A Wing and A Prayer, which rewards the Bracers of Reverence. They have a nice amount of stamina and defense, coming up as a middle path between the other two bracers, and come with hit rating for grabbing more threat.
Near the tail end of the long quest chain started with They Took Our Men, you'll get a chance to watch a pretty epic storyline reach an ending of sorts. As a reward for watching that, you can pick up the Gauntlets of Vigilance, a nice set of gloves with plenty of strength and agility for dodging and parrying, as well as a good amount of stamina and defense.
If you need a bit more defense that those offer, though, or just haven't had the time to finish that quest chain, consider getting a Blacksmith to make you some Daunting Handguards. They don't have near as much strength and not quite as much stamina as the Gauntlets of Vigilance, but they have twice as much defense rating.
Do you see a pattern yet? Tempered Saronite comes to the rescue once again. The Tempered Saronite Belt is going to be a quick and easy upgrade that has all of the basics to start tanking.
For a more a high powered belt, you'll want to keep an eye out for the Girdle of Obscuring in the Oculus, which doesn't have as much defense rating as the Tempered Saronite, but does beat it rather handily in all other areas, and adds some parry rating besides.
For the questing option, you'll want to check in with your friendly neighborhood Icecrown airship Death Knight again. The Gutbuster of Aldur'thar does lose out to the Tempered Saronite Belt on armor and defense rating, but edges it out in strength and stamina by a decent margin. It's a reward from the Not A Bug quest.