Is It Possible To Still Upgrade To OS X 10.8 Mountain Lion?
Yes, Apple sells older OS as redeemable downloads from the web store.

Apple sells OS X Mountain Lion redemption codes, alongside OS X Lion, for £14 each on the UK store. 

The US store link is below and it appears the part number is the same no matter which store you select:


http://store.apple.com/us/product/D6377Z/A/os-x-mountain-lion
http://store.apple.com/us/product/D6106Z/A/os-x-lion