Well versed Alchemist are always treasured in any serious raiding guilds for their valuable potions, elixirs and flasks and more so with the latest WotLK expansion. As regularly mentioned, Herbalism profession would be the perfect companion to lower the cost of powerleveling Alchemy.

Once again, let's have a look at where the trainers are located:

Alliance:
Lanolis Dewdrop - 58,62 in Valgarde, Howling Fjord.
Falorn Nightwhisper - 57,71 in Valiance Keep, Borean Tundra.

Horde:
Arthur Henslowe - 41,54 in Warsong Hold, Borean Tundra.
Wilhelmina Renel - 78,28 in Vengence Landing, Howling Fjord.

Neutral:
Linzy Blackbolt - 42,32 in Dalaran.


Here is the powerleveling guide for Alchemy 375 to 450. Kindly note that some of the steps are overlapping, so use the one which you have mats for or can be bought cheaply from the auction house:

375-380
Pigmy Oil - 1 Pygmy Suckerfish

380-385
Potion of Nigthmares - 1 Goldcloves, 2 Taladra´s Rose, Imbued Vial

385-390
Elixir of Spirit - 2 Taladra´s Rose, Imbued Vial

385-395
Elixir of Mighty Strength - 2 Tiger Lily, 1 Imbued Vial

395-405
Elixir of Mighty Agility - 2 Goldclover, 2 Adder´s Tongue, 1 Imbued Vial

395-415
Indestructible Potion - 2 Icethorn, 1 Lifebloom, 1 Imbued Vial

410-415-422
Runic Mana Potion - 2 Lichbloom, 1 GoldCloves, 1 Imbued Vial

400-420-425
Migthy Alchemist Stone – 20 Adder´s Togue, 15 Tiger Lily, Frost Lotus
Mercurial Alchemist Stone – 20 Goldclover, 2 Eternal Life, Frost Lotus
Indestructible Alchemist Stone – 20 Deadnettle, 12 Pygmy Oil, 1 Frost Lotus

425-435
Earthsiege Diamond – 1 Darkjade, 1 Huge Citrine, 1 Eternal Fire

430-440
Skyflare Diamond – 1 Bloodstone, 1 Chalcedony, 1 Eternal Air

435-450
Flask of Endless Rage - 7 Lichbloom, 3 Fire Leaf, 1 Frost Lotus, 1 Enchanted Vial
Flask of Pure Mojo - 7 Icethorn, 3 Pigmy Oil 1 Frost Lotus, 1 Enchanted Vial
Flask of Stoneblood - 7 Lichbloom, 3 Cristallized Life, 1 Frost Lotus, 1 Enchanted Vial
Flask of the Frost Wyrm – 5 Icethorn, 5 Lichbloom, 1 Frost Lotus, 1 Enchanted Vial