After Apple cancelled the hardware support for the Clamshell iBook which means that new parts are no longer produced, there are only a few sources of supply left. The first parts you have to replace are the battery, keyboard and power supply, but there are more parts you will need to keep your iBook up and running.
The largest second-hand market can be found on Ebay. The cheapest way is to buy a similar Clamshell to have a few parts in stock that you will need in the future.
A well-stocked, but higher priced assortment is available especially from US and UK merchants. Here comes an international link list:
The BookYard (UK)