Getting mountain lion
http://store.apple.com/us/product/D6377Z/A/os-x-mountain-lion?find=mountain+lion

From the Apple Store!  (Not the App Store...)

Although, from my understanding, there are no Macs that can run Mountain Lion that cannot also run Mavericks.  Mountain Lion dropped support for some older models (with older graphics cards on the whole), but since then support remains the same thus far.