I can’t find it anywhere.
You can buy Wavelab 8 full version, and then download Wavelab 6 (i assume you want the full version of Wavelab 6). The full version Wavelab 8 license will support full version Wavelab 6.
Wavelab 6 available on the downloads page, under “Unsupported Products”:
Then you have Wavelab 6 and Wavelab 8 on the same machine (with all the other stuff Wavelab 8 has, like DDP, etc).
Don’t know about lesser versions (like elements, essential, etc)