If you're using a PowerPC Mac it's quite easy to get. Download the ROM Update for an iMac G3 from support.apple.com. You should be able to find a freeware tool that can open the package and get the ROM file, but I'll be very surprised if you find a tool of that type that runs in anything other than Classic (hence the need for PowerPC).
Thats Basilisk II you have running there not SheepShaver. And did you initially mean a Mac OS 8 System Folder? Or ROM? Cause a ROM is a hardware file not a version of Mac OS. I figure you meant System Folder.