You may be able to talk apple in selling you the Leopard dvd.
Restore Tiger 10.4 & Leopard 10.5 DVDs are available from Apple by calling 800-767-2775 as of January 20, 2013.
Why do you wish to upgrade?
Safari 4.1.3 for Tiger
iCab - The Taxi for the Internet
tenfourfox -- It's a port of the latest FireFox to run on older hardware and software.
"World's most advanced web browser. Finely tuned for the Power PC."
alternative download site: