How do I go about buying a Harddrive?

I have an ibook G4 that I bought a few years back. I guess I did not treat it very special and when I run disk utility it says “keys out of order, rebuilding catalog B-tree”. which means i need a new harddrive. Where can i go to buy a harddive what my computer will support? and what should i “look for” in a harddive other than size?

Obviously I left a lot of details out… but i will be happy to answer any questions if you think i dont need a harddrive.