Updating OSX Leopard to OSX Lion

I want to update my MacBook Pro (early 2008) to OS X Lion and it's running OS X Leopard. I know that in order to do so I need the Mac App Store which was released in OS X Snow Leopad. I've looked around the Apple site to buy the OS X Snow Leopard software and be able to download OS X Lion from the App Store but can't find a way to get it.

I would appreciate any help that would let me know where I can find the software.