Backpacks can be bought at: The Outpost, 7 for 200 gold, Ashenvale, 10 for 500 gold, Torrentina,14 for 1000 gold, Sorrah, 17 for 2000 gold, The Gazebo of Doom, 35 for 400,000 gold, or 45 for 10,000,000 gold. See Shop Items.

Note: There is a mirror quest (Quests) where you can expand you equiped backpack 5 slots.

It would be a good idea to not complete that quest unless you have a 45 backpack, as the quest is limited to one per charecter.