There are currently four factions that you can champion in the game, and all of the tabards for each are available at Friendly. So you'll need at least friendly with all of the factions -- Wyrmrest Accord, Knights of the Ebon Blade, Kirin Tor, and the Argent Crusade -- before you go around to pick them all up. Getting there isn't hard at all -- if you've leveled by questing, you should have built up the necessary reputation (the one exception in my case is the Knights of the Ebon Blade -- I'm neutral right now, but I haven't really done any questing in Icecrown yet at all).
After the jump, a list of where you need to go to get each tabard.
This is very much a Dragonblight faction -- doing the Wrathgate quests (which you should have done anyway, they're aweseome) and any other quests you find in the big Dragonblight zone should get you up to Friendly. Tabard of the Wyrmrest Accord is available from their Quartermaster in Wyrmrest Temple, Cielstrasza, who totally is a dragon, for 1g at Friendly.
Knights of the Ebon Blade
Dutchess Mynx sells this tabard for 1g at Friendly, but in order to have access to her, you first need to complete the Shadow Vault quests in Icecrown. The chain starts off with "It's All Fun and Games," and running it to finish will not only let you see some great phasing action in one of the last instances in the game so far, but probably rep you right up to Friendly anyway.
Unfortunately, the old Tabard of the Argent Dawn that we got from the world event doesn't work in this case. You need to upgrade to the Tabard of the Argent Crusade, available from their quartermaster in Icecrown. You'll find him at the Argent Vanguard camp to the southeast.
This is the faction of the old mages in Dalaran -- if you need rep to get to Friendly, you can find them scattered around the world, or you can do Dalaran dailies, including the normal Dungeon daily quests. You can find the quartermaster in the city's Violet Citadel, and he'll sell you the tabard for 1g when you've got the rep for it.
There you go -- you can run around and get one or all of them, and then whenever you don one of the tabards, you'll get a buff that lets you gain reputation for the faction of your choice when running the endgame and heroic instances. It's probably a good guess that future patches will add in new factions to champion, so as soon as you hit 80, it's probably a good idea to run around and grab the tabards so you have them whenever you head off to an instance.
Now if only they'd give us a tabard closet to keep all of these things in...
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