I upgraded from Sonar Studio 8.5.3, to X3 Studio, so obviously I missed the PX-64.  Please tell me there's a way I can buy that separately! 

Sorry, it looks to be a Producer exclusive.

Reinstall Sonar studio 8.5.3 (you can keep 2 different Sonar version at the same time), copy px-64 folder and all its related content (dll and everything and save them where you want), uninstall Sonar 8.5.3, then paste what you saved before (px-64) in the new X3 studio vst folder and see if it works.

You will have to paste the folder in the same directory that you got it from in 8.5.3

Bagongolo
Reinstall Sonar studio 8.5.3 (you can keep 2 different Sonar version at the same time), copy px-64 folder and all its related content (dll and everything and save them where you want), uninstall Sonar 8.5.3, then paste what you saved before (px-64) in the new X3 studio vst folder and see if it works.

cowboydan
You will have to paste the folder in the same directory that you got it from in 8.5.3

 
As has already been stated in the first response, PX-64 wasn't included in 8.5 studio, which is what the OP said that he has.

Thanks for clearing that up, Paulo.  So, bottom line, I'm assuming is I can't get it separately.  Also, I'm assuming only AD2 is what I'll have to upgrade to?

King Conga
  Also, I'm assuming only AD2 is what I'll have to upgrade to?

I'm not sure what you mean by that exactly, but unless things have changed Addictive drums isn't included in Studio edition.