Accessing the Apps Store from 10.5.8
The way I interpret the "Requires 10.6.6" requirement is that any of the 10.6 releases (currently 10.6.6, 10.6.7, 10.6.8) are supported, along with any of the 10.7 releases (currently 10.7, 10.7.1, 10.7.2, 10.7.3). It doesn't matter which version you start or end with; just end up with 10.6.6 or newer.

Apple only sells SnowLeopard (10.6) on DVD. I'm not aware of a legal download for SnowLeopard. I recently bought the DVD for a relative and it had an older version, like 10.6.3. I installed from the DVD, then updated to 10.6.8 with Software Update. (My assumption is that Apple only updates the OS on the packaged media when they deem it necessary.)