The iBooks were last updated April 2003 to include the 900 Mhz model. Ironically they were the first 2003 machines that actually are Mac OS 9 bootable!
The second set of machines that came out in 2003 that are Mac OS 9 bootable are the non-Superdrive eMacs.
Still left over new and Mac OS 9 bootable are some 1.25 Ghz G4s on Apple's Store, (scroll below the software), the price has dropped to $1299 with custom building configurations more expensive, and the Titanium 15" Powerbooks.