Other World Computing, known more for its disk drives and other Mac-friendly hardware, also sells used computers, and they have great deals on iBooks here. I'd avoid the ones with broken CD drives, but for $275 to $599, there are some great deals -- most even have the Airport card included. The lower-priced models are iBook G3's, and the $600 iBook is a G4, just showing how these iBooks have really held their value.
I ordered a 300 GB drive from them recently. It came with ALL cables (FireWire 800, FireWire 400, USB) and was a very good value. I'm using it with Superduper to backup my mom's G5, and all has worked perfectly. So I do trust them to deliver a good product.