Another place to ask is on a local Freecycle list. While some people still pay for G3 Macs, they are quite old and you may stumble across someone who has one tucked away in the basement who would gladly give the machine to someone who would use it. If you are able to repair computers, it is worth mentioning that you are able and willing to fix it. If you are only able to fix simple problems, like installing software or replacing hard drives, just ask if the computer turns on and they can see something (anything) on the screen.

If you want to run Mac OS 9 or earlier, it may also be a good idea to ask about pre-Mac OS X software. I would imagine that there is a lot of pre-Mac OS X software that is hanging around and being unused since it won't work on Intel Macs.