Is there a legal way to download old installation discs for free?

A few years ago I bought an old Mac, PowerPC I think, but it doesn't work properly. If I boot normally only one of the two CPU cores works, but both work in safe mode.

I imagine that there are ways to repair it, but as it's second hand, I would probably be better off with a clean installation. I've never owned a Mac before, so although it's outdated, I'd rather get it back to a default state than install Linux, and it also means that once I've finished with it I can pass it on to someone else who would appreciate it.

Is there a way to get the installation media? From what I remember from the last time I tried to get it running, it can use either Snow Leopard or Mountain Lion (10. something?)

Thanks in advance :)