Where can I get a disc to reinstall?

I have two older macs, an iMac g5 and an old PowerBook g4 titanium. Both still work quite well (though the PowerBook does need to be plugged in at this point) and I would like to clear them off and reinstall them fresh so I can give to my younger cousins. I never had discs for them, where can I get the discs to be able to do this? I believe I need leopard and tiger. Thanks for your help!