OK, thanks for the addition information.
Installer discs are bootable, since they do have a minimized version of Mac OS on them with a System folder. I thought you meant just the System folder.
Many of those older PowerPC Macs had very specific install discs that came with them, due to the fact that some of the hardware required exact graphics and System drivers. Because yours has the ATI graphics instead of NVIDIA, it would have had a specific OEM install disc with it. Hopefully the copy the previous owner supplied was that version, and was created as a bootable disc.
I t will be tough to find, but you're going to need to find a FireWire HD that has Mac OS 9.2.2 or OS X 10.3-10.4 already installed on it that you could use as a startup drive, or access to a FireWire CD/DVD drive. It's a bit foggy, but I don't remember for sure whether the eMac will startup from a FireWire optical drive. A FireWire HD would have to be formatted as Apple Partition Map, not GUID like the Intel Macs for the eMac to startup from it.
Do you happen to have a Mac user group near you? Someone there may have some older Apple/Mac hardware/software that could help you out.
C