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Any PC store will have drives working with your iBook.

The drive has to be a 2.5 inch ATA, not SATA!
 
If you can get to the city then Capitol Techonology Center next to the Capitol Theatre down in Haymarket stocks ATA drives for your iBook.

They seem to be the cheapest in Sydney.
 
Have you checked newegg.com? You can get the WD 320gb IDE/ATA hard drive for under $100.

Also be sure to first check out www.ifixit.com for guides on how to change out the HDD on the iBooks so you know what you are getting yourself into :p. It's not just taking out one screw on the bottom and removing a cover like on many PC laptops ;).
 
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