Not sure if this will work, but your ibook should be able to start up as a firewire drive (assuming your imac has firewire). I don't know if OS9 can read a target drive comp, though.
Otherwise, you'd need cds. You can get them for $10 from Apple if you have a registered DVD version of OSX. I highly suggest not putting 10.4 on an imac G3 333mHz anyhow, as it will run slow as hell. The best OSX for that machine would be, in my opinion, 10.2, though 10.3 will run ok on it (I have 10.3 on a G3 400mHz imac and it's a little slow, but ok)...