Jerkin of the Untamed Spirit (Chest: 425 Armor) -- This beauty is available as early as 58 if you can manage the Hellfire quests that early or get a friend to help! You'll find it is invaluable for tanking during the leveling process, but as you come closer to level 70, you'll be hoping to find something better. Horde druids have to follow a bounty from a wanted poster at the entry to Spinebreaker Post, burn some buildings, "ensorcel" an eyeball, and finally defeat a nasty badguy, while Alliance druids just have to burn some towers for Wing Commander Dabir'ee near Zeth'Gor in Hellfire Peninsula.
As for an upgrade, you can consider doing a couple quests at minimum level 65 for Nickwinkle's Harness Experiment (343 Armor, but with lots more stamina - Alliance only), or buying a Chestguard of the Talon (378 Armor, but with better agility and strength) at level 68. At 70, you may want to see if you can ask (or beg!) a leatherworker to help you with a Heavy Clefthoof Vest (500 Armor!) -- although I must admit I haven't seen many leatherworkers around lately, so this might be pretty tough to get.
Cloak of the Valiant Defender (Back: 262 Armor) -- If you don't happen to find the Bogstrok Scale Cloak from Rokmar the Crackler in the Slave Pens while you level, you can also consider getting this cloak from a series of 8 quests in the Netherstorm at minimum level 67, starting with Ravandwyr in Area 52.
At level 70, you can look forward to finding a Thoriumweave Cloak (390 Armor) from Mechano-Lord Capacitus in the Mechanar. This is the single highest-armor cloak in the game, although you may find other cloaks with similarly high armor and additional resilience, such as the Resolute Cape and the Gilded Thorium Cloak.
Supple Leather Boots (Feet: 352 Armor) -- These boots will be the best you can get for tanking for a while. To pick them up, all you have to do is reach level 66, visit Timeon at Ruuan Weald in the northeastern Blade's Edge Mountains and do two quests for him. These quests will take a few minutes of your time fighting arakkoa, but they're not very hard.
And now that patch 2.1 is here, you will be able to upgrade these with some buffed up Heavy Clefthoof Boots (394 armor), now available through leatherworking.
Ash Tempered Legguards (Legs: 475 Armor) -- These are a bit more difficult to get, but still nothing you can't do on your own. To begin, head over to Shadowmoon Valley at at least 67th level, and seek out Gryphonrider Kieran at Wildhammer Stronghold for the Alliance, or Sergeant Kargrul at Shadowmoon Village for the Horde. Either of these will send you off looking for some journal pages that point to an arakkoa deserter, named Parshah. Once again, Parshah will have you busy killing other arakkoa and collecting things, but once you have thwarted the dark conclave of evil bird men, you should be able to get your spiffy tanking leggings.
Once again, with patch 2.1, you can upgrade at level 70 to the Heavy Clefthoof Leggings (503 Armor) if you know a friendly leatherworker or are willing to pay some significant gold at the auction.
Manimal's Cinch (Waist: 348 Armor) -- Once you reach level 68, it's time to start thinking about this quest chain called the "Cipher of Damnation." This quest is notoriously long, and it eventually gets rather difficult, but at this stage it's not too bad yet. Once again, there are quest-givers at each of the two outposts in Shadowmoon Valley (Earthmender Sophurus for Alliance and Earthmender Splinthoof for Horde), who will ask you to go find Earthmender Torlok and help him try to make amends with the tortured elementals of Shadowmoon Valley by... killing them and collecting their souls. Once you've done this somewhat incongruous task for the spirits of Earth, Fire, Water and Air, Manimal's Cinch will be yours, and you will be free to move on to the next stage of this quest, involving Oronok Torn-Heart.
After this, there are only two leather belts I could find with a higher armor rating, the Tree-Mender's Belt (406 Armor) from the last boss of Heroic Hellfire Ramparts, and the Belt of Natural Power (423 Armor) -- another top-end crafted item that might make feral druids everywhere consider dropping their current professions and taking up leatherworking.
Umberhowl's Collar (Wrist: 281 Armor) -- After doing more of this and that, the Cipher of Damnation quest splits into three parts. The one you'll want to focus on is the quest for the third fragment of this damnable Cipher. Borak, son of Oronok, will eventually ask you to beat up this dragon-riding elf named Ruul in southern Shadowmoon Valley, and you'll need to bring some friends along for that battle.
As far as I know, this is the highest-armor leather bracer in the game. The next one down from this are the Forestheart Bracers (237 Armor), which drop off of Warchief Kargath Bladefist in Heroic Shattered Halls.
Verdant Gloves (Hands: 393 Armor) -- The final item for today is this reward for a series of quests that introduce you to Akama, the would-be betrayer of the Betrayer in the Black Temple. The quest starts in Shadowmoon Valley, either with Anchorite Ceyla at the Altar of Sha'tar for those who chose the Aldor, or Arcanist Thelis at the Sanctum of Stars for the Scryers; both of these ask you to collect tablets in the Ruins of Baa'ri. This quest eventually ends up with Akama asking you to fight a difficult battle on a terrace outside the Black Temple itself, so be ready to bring some friends along for the culmination!
After these gloves, the next-highest armor rating for your hands comes from the tier 4 feral Gauntlets of Malorne (475 Armor)! So... these should hold you over for quite a while.
There are many other items with high armor ratings available, including notably some high-armor weapons, such as Braxxis' Staff of Slumber, the Greatstaff of the Leviathan, and the Earthwarden. Some trinkets like the Badge of Tenacity deserve special mention also, as well as the Smoking Heart of the Mountain and the Mark of Tyranny, for being two druid tanking trinkets from before the expansion that are still useful to this day. These, however, do not come from plain old Outland quests, so we may cover how to acquire them on another day.
Funny bear picture by the famous druid named Alamo, who "teechs u 2 play DURID!"