There are 3 specialty cloth cooldowns in TBC (similar to Mooncloth in Classic): Spellcloth, Shadowcloth, and Primal Mooncloth. The crafts for these are on separate 4 day cooldowns, i.e. you can make one of each every 4 days (actually 3 days 20 hours).
Crafting these cloths requires tailoring 350, which requires level 50. You can specialize in one of the three types of cloth, which gives you 2 cloth per craft instead of 1. Specializing requires level 60 and tailoring 350.
These cloth will be in high demand - most casters will need 20-40 cloth, which would take a long time to make on just one character.
The crafts require being in specific locations:
Shadowcloth
Mats: Bolt of Imbued Netherweave, Primal Shadow, Primal Fire
Location: Altar of Shadows in Shadowmoon Valley (58.3 70.9). You can fly there if you have flying mount, or run through Netherwing Pass (62.4 62.4). There are lots of high-level mobs in the way, so the easiest way to get there as a low-level toon is to get a summon.
Spellcloth
Mats: Bolt of Imbued Netherweave, Primal Mana, Primal Fire
Location: Netherstorm
When you do the craft, it spawns a Nether-wraith (level 70 normal mob), which drops 1-3 Motes of Mana and Fire as a bonus. It's pretty easy to solo at level 70, or you can do the craft in town in Area 52 and let the guards help kill it. However, you only get the loot if you or your party deals at least 50% of the damage. If you don't want to bother with killing the elemental for motes, just summon it by the guards in town and walk away.
Primal Mooncloth
Mats: Bolt of Imbued Netherweave, Primal Life, Primal Water
Location: any Moonwell. Some convenient Moonwells are:
Least walking: Right at the FP at Evergrove, Blade's Edge Mountains
Closest to Shattrath: Cenarion Thicket, Terokkar Forest (45.1 23.6)
Stormwind or any other Moonwell in Azeroth
To conveniently get to these locations, you'll want the following flight points:
To Netherstorm for Spellcloth:
Shattrath
Telredor, Zangramarsh
Toshley's Station, Blade's Edge Mountains
Area 52, Netherstorm. Note: you can't summon inside Netherstorm, so to get there you can summon to bridge at the edge of Netherstorm, and then run to Area 52. The path is low mob density so relatively safe.
You can also just set your hearth in Area 52.
In addition, Evergrove, Blade's Edge Mountains has a convenient moonwell for Primal Mooncloth right next to the flight point - it's skippable since Toshley's connects to Area 52.
To Altar of Shadows for Shadowcloth:
Shattrath
Allerian Stronghold, Terrokar Forest
Wildhammer Stronghold, Shadowmoon Valley
Sanctum of the Stars, Shadowmoon Valley (Scryer's only - if you're Aldor you just have to trek across Shadowmoon Valley)
For Master Tailoring Trainer:
Honor Hold / Thrallmar
You can specialize in one of the three types of cloth, which gives you 2 cloth per craft instead of 1, and gives you access to craft and wear the corresponding set: Spellfire / Frozen Shadoweave / Primal Mooncloth. Specializing requires level 60 and tailoring 350.
You can only specialize in one type at a time, although you can change your specialization at any time for a gold cost - 170g at level 70 (150g to unlearn and 20g to learn), cheaper at lower levels - 70g at level 60. To change your spec, talk to your current specialization's vendor in Shattrath City to unlearn your current specialization, then talk to another to learn the new specialization. You do not have to complete the specialization quest when changing your spec. Note that you will lose any specialization specific recipes when you unlearn a specialization, and the specialization-specific gear will not function while you are a different spec, same as if it's red durability (the pieces say e.g. “Requires Spellfire Tailoring”).
Especially on lower level toons where the respec cost is cheaper, it can be quite profitable to change specializations every cooldown cycle to get more extra cloth. Note that you can do your crafts in reverse order every 4 days to save a respec, e.g. if you want 2x of both Spellcloth and Shadowcloth, you can craft Spellcloth as Spellcloth spec, respec Shadowcloth to craft Shadowcloth; then 4 days later craft Shadowcloth still as Shadowcloth spec, and then respec back to Spellcloth to craft Spellcloth.
To learn a tailoring specialization, talk to the tailoring specialization vendor in Shattrath City, who will give you a quest. The quest basically requires you to go to the same place where you do the craft. The quests for Primal Mooncloth and Shadowcloth specializations simply require going to a Moonwell or the Alter of Shadows and using the quest item. For the spellfire specialization quest, you have to go to Netherstorm, kill a Furious Nether-wraith (level 70 normal mob), and loot its quest drop. Low level characters will need the help of a high level character. You can kill it in town with the guards’ help but you or your party need to deal at least 50% of the damage to get kill credit so you can loot the quest item drop; be aware the guards take it down pretty fast on their own. If you fail to get the kill for the quest item, you can just resummon it and try again. You can also just start with another of the specializations and respec.
Spellcloth is likely to be most in demand, which means both the cloth and the primal mats for it (fire and mana) are likely to be more expensive compared to others. So specializing in spellcloth is likely to be most valuable, and I expect you can trade extra spellcloth for shadowcloth or primal mooncloth at a substantial profit.
To train tailoring past 300 you'll need to go to your Master Tailoring Trainer - Hama at Honor Hold for Alliance, Dalianna at Thrallmar for Horde. You'll also need to go to Shattrath City for the tailoring recipe vendors and mana looms. You can get there by summon/port if you're below level 58.
Here is a typical skilling route. For alts that you're just using for the tailoring cooldowns, you can use the same crafts and just stop at 350 - use the extra bolts for any characters that need to get to 375.
300-325 Bolt of Netherweave (trainer). You'll need tons of bolts for later crafts.
325-340 Bolt of Imbued Netherweave (vendor recipe - sold by Eiin in Lower City, Shattrath, with unlimited stock). Must craft at a mana loom - there's one right next to Eiin.
You'll need 90 for 360-375. If you don't need more bolts, you can start making the next craft a few points early, from 335 or above.
340-345 Netherweave Boots (trainer). Available at 335, can start earlier if you don't need that many Bolt of Imbued Netherweave.
345-360 Netherweave Tunic (vendor recipe - Eiin, Shattrath)
360-375 Imbued Netherweave Tunic (vendor recipe - Arrond in Shadowmoon Valley. You need to have a neutral Scryers reputation to buy from him, but you can also get someone else to buy it for you or buy it on the AH since it isn't BoP)
The last few green points for the bolts take a lot of attempts, but you'll need far more bolts anyway to get to tailoring 375, so you might as well craft them for free skillups. You can switch to an alternate craft for the last few points if you don't need that many bolts.
Leveling 300-350 takes roughly 1,000 Netherweave Cloth (50 stacks), and 300-375 takes roughly 2,800 Netherweave Cloth (140 stacks).
Alternate options include bags which you can sell, and recipes that use Runecloth/Mooncloth if you can't find enough Netherweave:
Mooncloth: green 305, gray 320
Mooncloth Bag or Runecloth Shoulders or Wizardwave Robe: orange 300, yellow 315, green 330, gray 345
Netherweave Bag: orange 315, yellow 320, green 330, gray 340
Imbued Netherweave Bag: 340-350, yellow 340, green 345, gray 350