The first thing I'd like to point out is that you don't need to do what I did while getting pictures for this article and run around like a headless chicken collecting tabards from all corners of the world. The easiest way to re-collect any tabards you've previously owned is by visiting Elizabeth Ross in Dalaran or a tabard vendor in any major city. I do need to mention that they sell tabards for the currency each faction uses (e.g. you still have to pay 10 Glowcaps to purchase a Sporeggar tabard from them) so if you don't have the currency on you at the time, you'll still be making a field trip to collect it.
For this guide I'm assuming that you do not necessarily have access to now-unobtainable tabards like Competitor's Tabard or Tabard of the Protector, so if you do have pieces like this sitting around your bank, you're just that much closer to the achievement. As mentioned previously, you can fudge Twenty-Five Tabards by getting TCG tabards like Tabard of Nature or Tabard of Frost, but I'm also assuming you don't have access to any of these. This achievement is very much doable with a little hard work and a pinch of luck, so let's get rolling.
Your first tabard should be easy enough -- purchasing a Guild Tabard, even if you're not in a guild, still counts.
1 down, 24 to go! And the biggest hurdle to surpass is the BC tabard set --
Burning Crusade faction tabards:
In contrast to Wrath factions, you will need to be exalted with BC factions in order to purchase their tabards, so count on getting to exalted with all of the following. In order do the heroic version of dungeons to which their reputation is linked, visit their quartermasters (I've included names, Wowhead and Wowwiki links, and coordinates where relevant) and purchase the keys needed to "unlock" heroics. Depending on your class, most or all BC-era heroics are soloable at 80 with decent gear, but if you're a cloth class or anyone who runs the risk of getting stomped by large pulls, find a buddy or two who needs to work on the same reps you do and pair up.
Bear in mind that this is not meant to be an exhaustive list on how to farm rep for each BC faction, as that's way beyond the scope of this article. My intent here is to summarize the most efficient means of getting to exalted with each if you're approaching the rep grind at 80. For more information on your options, please visit the series of links I've included with each entry.
2. Honor Hold (Alliance) or Thrallmar (Horde).
Get exalted by exhausting their quest offerings (Honor Hold for Alliance, Thrallmar for Horde) and running normal Shattered Halls or heroic Ramparts, Blood Furnace, or Shattered Halls. Shattered Halls, while rewarding the best rep, can be a tall order even for a tank class at 80 due to the size of the pulls and the amount of damage they dish out, but I take a vicious pleasure in rolling this place nowadays -- it was hell on earth for a rage tank back in the day.
Honor Hold quartermaster: Logistics Officer Ulrike at 56,62 in Honor Hold (Hellfire Peninsula).
Thrallmar quartermaster: Quartermaster Urgronn at 55,40 in Thrallmar (Hellfire Peninsula).
3. Cenarion Expedition
CE rep is somewhat interesting, as you can also farm it in Wrath content courtesy of the small outpost in Borean Tundra. In BC content, you can get exalted by doing their quests and running normal Steamvault or heroic Slave Pens, Underbog, or Steamvault (if you do Underbog, save any Sanguine Hibiscus you get for Sporeggar rep later). After turning in Orders from Lady Vashj (a random drop from Steamvault mobs), you can also turn in Coilfang Armaments (which are BoE) to Ysiel Windsinger at the Cenarion Refuge in eastern Zangarmarsh for 75 rep each. The Armaments drop randomly from mobs in heroic Slave Pens, Steamvault, and Serpentshrine Cavern (a 25-man raid), and they're nice boost on top of whatever rep you'll be getting from killing the mobs inside.
If you have difficulty soloing or getting runs for the Coilfang dungeons and nobody's putting Armaments up on the auction house, you can plug away on the repeatable quest Can't Get Ear-nough in Borean Tundra for 150 rep a pop. Doing the dungeons is definitely faster and much more lucrative, however.
Cenarion Expedition quartermaster: Fedryen Swiftspear at 79,63 in Cenarion Refuge (Zangarmarsh).
Everyone's favorite mercenaries, and with a pretty cool-looking tabard to boot. Get exalted by exhausting their quest offerings and running heroic Mana-Tombs, grinding ogres in Nagrand for Obsidian Warbeads, using Ethereum Prison Keys on the cells in Netherstorm, and...hell, there are a lot of ways to get Consortium rep, now that I think about it (so please do visit the Wowwiki page I linked above for a more in-depth guide to it). The fastest way is definitely to grind heroic Mana-Tombs, but if you're also working on Kurenai/Mag'har rep, you can do a twofer by grinding Nagrand's ogres. The kills themselves will net you reputation for the Kurenai/Mag'har, and you can save the Warbeads for Consortium turn-ins with Gezhe (31,58 in the Consortium camp in Nagrand).
Consortium quartermasters: Karaaz at 43, 34 in the Stormspire (Netherstorm) or Paulsta'ats at 30,57 at the small Consortium camp in Nagrand. Please note that the key you need to access heroic Mana-Tombs is sold by the Lower City quartermaster (see below), as it's an Auchindoun instance.
5. Lower City
I feel obligated to note that the Lower City tabard tends to match druid armor well, or anything that tends to have a dominant green/brown color scheme. Get exalted by exhausting their quest offerings and running normal Shadow Labyrinth or heroic Sethekk Halls, Auchenai Crypts, or Shadow Labyrinth. Shadow Lab is, like Shattered Halls, potentially a tall order even for an 80, but pretty lucrative. You're also likely to get a few drops that help you with Aldor rep (see below).
Lower City quartermaster: Nakodu at 62,69 (Shattrath).
6. Kurenai (Alliance) or Mag'har (Horde)
Experienced BC players are probably undergoing flashbacks to hours of grinding in order to get their talbuks, but hey -- at least these factions give you a reward you can still get some use from at 80. Talbuks are still among the coolest and best-looking mounts in the game, although I've always wondered if they're Blizzard's riff on Yakul from Princess Mononoke. Get exalted by exhausting their quest offerings (Kurenai for the Alliance and Mag'har for the Horde) in Nagrand and then grinding ogres, both for the reputation you'll get from the kills and their Obsidian Warbeads (although, as noted earlier, you can also use Warbeads for Consortium rep, so plan ahead).
Kurenai quartermaster: Trader Narasu at 55,75 in Telaar (Nagrand).
Mag'har quartermaster: Provisioner Nasela at 53,37 in Garadar (Nagrand).
7. Keepers of Time
One of the things that's never failed to annoy me about the Bronze Dragonflight is just how many indignant sub-factions keep demanding hours of my time before they'll believe my main can be trusted. I'm exalted with the Brood of Nozdormu, Keepers of Time, and the Scale of the Sands, and I should not have to keep grinding bug parts or running Hyjal or saving the damn time-stream in order to prove to these screwballs that I'm not a psychopath. If there's another sub-faction of pissant Bronze dragons in Cataclysm, my sanity is going to snap and I will murder every last one of them that I can find. Maybe they already know this (being time-travelers and all) which is why I keep getting subjected to such miserable grinds.
If I recall correctly, this tabard wasn't present when BC went live, and was added several months to a year in. Get exalted by running normal Black Morass or heroic Durnholde Keep and heroic Black Morass.
Keepers of Time quartermaster: Alurmi in the Caverns of Time (Tanaris). Coordinates are useless here because the Caverns are largely underground, but she's located about a stone's throw away from the repair guy around the small collection of buildings in the tunnel that empties into the central Caverns area. That probably doesn't sound like it makes much sense either, so try this: after you get off the drake that flies you down into the Cavern, hop on your mount and backtrack over the path the drake flew you in, until you see a bunch of vendor NPC's. She's there.