You can download the iso from:

http://www.macbits.com/macppc.iso

I don't know if it will work for you or not. I would try setting your 
Open Firmware boot-device to the CD-ROM path and then just issuing boot.

On my beige rev. 1 G3 I had to set boot-device to ide0/disk@0:0 and it 
started but wasn't able to complete booting. You may have better luck. 
I was able to boot from the floppy though so you might try making the 
boot1.fs and boot2.fs floppies from the iso listed above with dd and 
trying that. Apparently you need both floppies because the boot files 
are split between the two floppies. I put in the boot1.fs floppy and 
started the boot and then it asked for the boot2.fs floppy. I am aware 
that the beige rev. 2 G3 isn't supposed to be able to boot from the 
floppy but you ought to try it with these new files.

Bryan