Every 2.5 inch harddisc will work. But you don't want to do this on your own. Let professionals do this, since it's a real mess to open the case without special tools.
And welcome to the forum, Michi!
thx for welcome. I'm a x86 Linux user, now I got an iBook.
everyone here tells me not to do things by myself... but I got a broken (see http://www.macosx.com/forums/showthread.php?t=38869) ibook, it would cost about 300$ to repair it (they said they would change the whole display-unit), it'll get the broken part for about 20$... theres no warranty on this iBook, I have the special-tools (it's only a 1/16" allen key and a small screwdriver). So, why not do it by myself!?!
I understand your situation. Just be warned. Apple books are not meant to be serviced by regular ppl. I read some posts how ppl desperately tried to fix things on their on and went into deep troubles.
Anyway, I wish you good luck, Michi!
And hey, there are ppl posting over 20 times every day and they are beyond 6000 posts... Am I really crazy?