You didn't say where you are located. In the US, call Apple at 800-MY-APPLE as they were recently reported to still have OS X 10.5 Leopard available for sale. You can't buy it at Apple stores anymore. If that doesn't work, another source is hardcoremac.com, then there's alway ebay. If you go ebay, make sure you get retail black-faced disks and not gray-faced system disks, as those are locked to the system type they are made for. If you have access to Craigslist, that might be another source.