John
The problem is 8.5 was an update and required the DVD of Sonar 8 to install. You could buy the 8.5 DVD but most didn't.
They are all in your store account.
I think OP is saying the plug he's looking for is on the 8 disc which is missing. So he would need Cakewalk to generate a download link for the entire Sonar 8 package and then the OP could do a custom/advanced installed to get that specific plug... in theory of course.
AFAIK the Bakers will generate a download link in such situations provided the customer can verify they did indeed own the legacy product. I may be wrong on that.
Still it doesn't hurt to contact support. Really that's the only way you'll be able to obtain such an old product legally (unless you can somehow purchase an old, surplus, unregistered version of Sonar 8 somewhere and even then I think that might be borderline skeevy).
Seriously... just get a hold of Cake Support.