Using a Mac, is there any way to purchase iPad-only HD content from iTunes?
Source: http://support.apple.com/kb/HT3209


  In order to purchase, rent, and view HD content from the iTunes Store
  on your computer, you need to have the latest version of iTunes
  installed, you can download the latest version here. In addition, you
  will need to have components that support HDCP (High-bandwidth Digital
  Content Protection) installed on your computer. 
  
  Mac System Requirements for HD content:
  
  
  Mac OS X v10.5 or later iTunes 10 or later
  2.0 Ghz Intel Core 2 Duo or faster processor
  At least 1 GB of RAM 
  A display that supports HDCP with a screen resolution of 1024x768 or
  higher.
  
  
  If your computer does not meet all of the above requirements, you will be unable to 
  purchase, rent, or view HD content from the iTunes Store. 
  
  Note: If you are trying to view HD video on an external display, the
  display must have a digital connection (DVI, DisplayPort, or HDMI) and
  also support HDCP.


If it isn't obvious where to buy the movies, I would guess that your system requirements are not there.