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
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.