If you’re still running Leopard, aka Mac OS X 10.5, you now have no Apple-provided way of getting either Lion or Mountain Lion on your machine. You will have to buy a Snow Leopard DVD — formerly $29, but now upwards of $70 on Amazon and eBay to update your system, and then you will have access to the Mac App Store to get Mountain Lion.
But, if your Mac can’t run Mountain Lion (requirements are here), then, uh, tough. You can’t buy regular Lion any more. You’ll have to track down the Apple Lion USB Thumb Drive installer (originally $69), which is quite rare. I’m sure Apple would be happy to interest you in a shiny new Mac to help bring you up to date, though.
MacBook Air 2011: updating from Snow Leopard
How to make a Lion installer USB stick when you already have Lion installed