Since we are talking about cheap and trustable companies you might want to check out OffGamers.com

World of Warcraft 60-Days Pre-Paid Game Card [EU] - 19.20 EUR
World of Warcraft Standard Edition Cd Key + 30 Days Play Time (No World of Warcraft Game Card Required) [EU] - 7.72 EUR
Burning Crusade Expansion Cd Key [EU] - 7.72 EUR
World of Warcraft Battle Chest Cd Key + 30 Days Play Time (No World of Warcraft Game Card Required) [EU] - 15.23 EUR
Wrath of the Lich King Expansion Cd Key [EU] - 20.83 EUR