For those Mac owners still on OS X Snow Leopard, does the possibility exist to upgrade to OSX 10.8 Mountain Lion rather than to 10.9 Mavericks?
Yes, Apple sells older OS as redeemable downloads from the web store.
The US store link is below and it appears the part number is the same no matter which store you select:
I have some good news for you. Every Mac running Mac OS X 10.6.8 (Snow Leopard) or later will be able to upgrade for free to Mavericks.
Check the specs page on Apple.com
But if you want to upgrade specific to Mountain Lion (can't think of a reason why?), the answer is no I'm afraid. Every time Apple releases a new OS, they drop support for the older ones.
You can always look on eBay or other second-hand sites to obtain a legal copy of the disk.