20. Stormpike Battle Tabard (Alliance) or Frostwolf Battle Tabard (Horde)
The former can be bought from Thanthaldis Snowgleam at 40,81 in the Alterac Mountains (outside the Alliance portal for AV) and the latter can be bought from Jekyll Flandring at 63,59 in the Alterac Mountains. You can also buy them from NPC's in the actual battleground, but you are likely to be busy at the time. Costing 60 Alterac Valley marks, these tabards aren't too tough to farm if you can just convince your team to play a little defense.
Ha ha! I kid because I love.
21. Silverwing Battle Tabard (Alliance) or Warsong Battle Tabard (Horde)
To be frank, I'm not fond of either of these, and at 60 Warsong Gulch tokens, you'll probably be spending a lot of time in WoW's most unpopular battleground. The silver lining? At least it's not linked to reputation the way the AB tabard is, because farming WSG rep is an absolute screaming horror.
The Silverwing tabard is sold by Illiyana Moonblaze at 61,84 in Ashenvale, and the Warsong tabard is sold by Kelm Hargunth, at 46,8 in the northern Barrens.
22. Arathor Battle Tabard (Alliance) or Battle Tabard of the Defilers (Horde)
You're going to have to get to either exalted with the League of Arathor (for Alliance) or with the Defilers (for Horde) before you'll have a shot at this one, and the quest to get it requires you to win an AB match while your team controls all 5 bases. You'll occasionally get lucky with a PuG on that, but a premade team's usually a better bet.
The Alliance quest is Control Five Bases; its Horde counterpart is Take Five Bases.
23. Tabard of Conquest (Alliance) or Tabard of Conquest (Horde)
I'm classifying this one as being slightly harder to get than the tabards associated with the older battlegrounds, if for no other reason than the fact that Isle's relatively new and a lot of players are still finding their feet there. If you already know what you're doing in Isle and have the good fortune to run into PuGs who also know what they're doing, Isle is a lot of fun. Also, this'll require you to win at least 100 Isle matches, as opposed to the minimum 20 and maximum 60 AV and WSG matches. The tabard is a reward for the meta-achievement Master of Isle of Conquest (Alliance) and Master of Isle of Conquest (Horde).
24. Tabard of Brute Force
This one is a reward for the Brutally Dedicated achievement, requiring you to win 300 ranked arena matches at 80. I await a series of catty and unhelpful comments on the ease of doing this on (class) versus (class). Nerf (someone else), buff (me), Blizzard you (censored) piece of (censored censored), etc., etc.
25. Relentless Gladiator's Tabard
Simple! All you have to do is get a team and personal rating of 2350 in 3v3 or 5v5, and then march up to an arena vendor and shell out 500 points. Easy, right?
PvE and/or quested tabards
26. Tabard of the Explorer
This one's potentially time-intensive depending on how thorough an explorer your toon was while leveling and questing, but it's pretty easy to get. Ride and fly everywhere you can on the map of every Azerothian continent and Outland, and bingo -- the tabard's in the mail.
27. Green or Purple Trophy Tabard of the Illidari
You can choose one of these two tabards as a reward from the Shadowmoon Valley quest Battle of the Crimson Watch (the prerequisite is The Journal of Val'zareq: Portends of War, which drops from Val'zareq the Conqueror, an elite Blood Elf NPC who patrols the Path of Conquest in central-southern portion of Shadowmoon). This is soloable at 80, and you should thank your lucky stars that you're not doing its original pre-nerf incarnation at 70, which was one of the most legendarily difficult non-raid experiences of its time.
28. Loremaster's Colors
If your toon is a fairly new character, don't count on getting Loremaster (one of the most intensive PvE achievements) quickly. If you've been playing for a while, some elbow grease (and a nice boost in 3.3 from some added UI functionality that Blizzard will be adding) can probably put you within range of the Loremaster tabard that's mailed to you upon completion of the meta-achievement.
Argent Tournament tabards
These technically belong to the last category, but I'm giving them their own because you can find them all in one place and the same stipulations apply to each. All of the following tabards cost 50 Champion's Seals and can be purchased after reaching Champion status with the "home factions" or (for the Silver Covenant/Sunreavers) exalted with the Silver Covenant/Sunreavers.
The "home faction" tabards didn't initially apply to the tabard count achievements, but that's been fixed by now, although you can't sell them back to the vendors in exchange for 50 Seals anymore either.
29. Argent Crusader's Tabard
Not to be confused with the reputation tabard at Friendly with the Argent Crusade, this one has to be purchased from Dame Evniki Kapsalis at 69,23 (the main tent at the Argent Tournament), and can also only be bought once you've attained Crusader status (exalted with all five home factions and through the Champion questline for each as well). I find this tabard to be incredibly convenient -- I can simply pop it on and port out for every ToC run. It's the sole piece that has to be purchased in a slightly different spot -- all of the following can be bought in your faction's main tent on the eastern side of the Tournament from the various quartermasters contained therein.
30. Darnassus tabard (Alliance) or Thunder Bluff tabard (Horde)
31. Stormwind tabard (Alliance) or Orgrimmar tabard (Horde)
32. Exodar tabard (Alliance) or Silvermoon City tabard (Horde)
33. Gnomeregan tabard (Alliance) or Sen'jin tabard (Horde)
34. Ironforge tabard (Alliance) or Undercity tabard (Horde)
35. Silver Covenant tabard (Alliance) or Sunreaver tabard (Horde).
Although we've already passed this time of year, waiting long enough will give you another crack at this:
36. Tabard of Summer Skies and/or Tabard of Summer Flames
One of these is a reward for completing the Shards of Ahune quest you can do every year for the Midsummer Fire Festival. While it's been reported that you can destroy your existing tabard and get credit for another one by buying the other at a tabard vendor's, I've always been too chicken to try it.
37. Tabard of the Scarlet Crusade
This is a 1-2% drop off the Scarlet Trainees who swarm to you like low-HP locusts after Herod's death in the Armory, and it shouldn't take you too many runs before it shows up, given that 20 of them attack. A word of warning, though; make sure you have all reflective damage abilities or buffs turned off, that your pet is on passive if you have one, and that you're not dropping a magma or elemental totem as a shaman -- the Trainees only have about 100 health, but you have to kill them yourself to be able to loot them.
If you're a Draenei...
38. Tabard of the Hand
This is a reward from the Bloodmyst Isle quest The Unwritten Prophecy and a large part of why leveling in the Draenei starting zones is so kickass.
If you're a Blood Elf paladin...
39. Blood Knight tabard
The envy of Alliance paladins everywhere, the Blood Knight tabard is a reward from True Masters of the Light, the last quest in the BE quest series to get their charger.
You only need to manage 25 of these for your brand new, spiffy Tabard of the Achiever, which will be mailed to you as soon as you get #25. Happy hunting!