You generally do not have a choice of format for movies given to you as a digital download from a Blu-Ray or DVD purchase. If you got an HD movie, that may be the only format available to you. Try this, though:
On the main iTunes Store page, sign into your account, go to Purchases under the Quick Links and then select Movies. Click the "Not On This Computer" button. Uncheck the "Download HD when available" box and you may then see SD copies of the movies and be able to download them.
If not, then you'll either have to get a device that supports HDCP - the Apple TV does - or repurchase the movies in standard definition.