Haris Pilton in Shattrath City has a 24 Slot Bag available for purchase at 3,000 gold, the Portable Hole! This 24 Slot Bag is the largest bag obtainable in Wrath of the Lich King Classic and will still have a lot of value if/when Cataclysm Classic releases after Wrath. In Cataclysm, there is a 26 Slot Bag that can be crafted by Tailors, but you can expect to pay much more than 3,000 gold.
Haris Pilton