I'm not sure they make one for the tray loading imacs, and if they do I have a feeling they are very expensive. As a matter of fact, Apple never shipped a combo drive that read both dvd and cd burner with any of the iMacs of that generation. Only dvd/cd read only and a cd burner without dvd player.
I think you can get these things third party somewhere still, but be prepared to spend. Cheaper to try to find a iMac G3 500 and up with a dvd/cd player and just firewire an external cd burner. The tray load iMacs don't have firewire so that's another issue.