I have Mac OS X 10.5.8 and I can't seem to upgrade to Mountain Lion. Are there any upgrades available to me? I've already updated.

Upgrade to Snow Leopard to be able to purchase Mountain Lion > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard Then, make a backup, insert the DVD and upgrade. Finally, go to  > Software Update and install the latest version.

http://www.apple.com/osx/specs If your computer is supported, open App Store and purchase Mountain Lion. Check if your programs are supported > http://www.roaringapps.com

...but apparently you have to have OS X 6.

Yes, and you can purchase the 10.6 DVD from this link

http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard