Targus offer several adapters suitable for Apple PowerBook and iBook laptops. Targus kits PA380E, PA171E and PA172E are compatible with Dell, HP, Sony and Apple products.
Alternatively, the TeleAdapt Inflight Power kit that BA sell on board should also be compatible with your iBook. It claims to suitable for all laptops and as most Apple Resellers stock this adapter too its probably a safe bet.
Hopefully someone can offer a more definite answer. After all the research I went into earlier this year, I ended up buying a back-up battery instead of an adaptor.