You're running Mac OS X 10.5.8 = Leopard.
Mac OS 10.6 is Snow Leopard and was available on a DVD to upgrade Leopard.
Apple no longer sell Snow Leopard AFAIK, though yiou may find a copy somewhere.
For Mac OS 10.7 (Lion) you need currently to have Snow Leopard installed to purchase the upgrade from the app store.
Lion is not available on a disk from Apple.
Apple will be releasing Lion on a USB stick or similar I believe, but it will be more expensive than an app store download.
Apple no longer sell Snow Leopard AFAIK, though you may find a copy somewhere.
Still available at the Apple online store at this writing: