You can buy a copy in either DVD or VHS from amazon.com
I recommend getting a copy on VHS. Believe it or not the film quality is much better on VHS. The DVD has a twenty minute portion where the subtitles are out of sinc. This doesn't happen on the VHS.
Plus, this is rather a pricy film and the VHS isn't as expensive as the DVD.
You can download it for free from avax.
sharethere's a stream of it on stagevu.com.
shareI don't believe stagevu has it. They've got 3 versions, no less, of French-dubbed "Le Beau-père"... better known under its original US title, "The Stepfather" (2009). A slightly different kettle of fish, no?
Just search for "beau pere 1981" to get working download links near the top of the results.
English (and other) subtitles are available, as usual, from http://www.opensubtitles.org/en/search/sublanguageid-all/idmovie-14331 .
The complete movie is available on youtube, in French. I used the "cc" option to be able to follow. The CC is faulty, and my French is nearly nonexistent, but CC was good enough to kind of know what was going on in each scene.