As far as I know you can't - I've looked everywhere. It is no longer available on HBO on-demand. fullmovies.com says they have it but once you pay the $29.95, it's not actually there.
There are a number of websites where one can watch a World War 2 documentary online. The YouTube video sharing website has a number of full length documentaries available for viewing. The video streaming websites NetFlix and Lovefilm have similar documentaries although both require payment to watch.
You can not yet watch it...
I usually watch documentaries at http://www.alldocumentaries.org/ (over 3000 free documentaries to watch and there are dozens new ones added daily)
Netflix
One can watch a David Guetta video at YouTube or at Myspace. Pierre David Guetta, or simply David Guetta, is a French house music disk jockey and producer.
There are several places online that one can watch Fahrenheit 911. These websites include Documentary Wire, YouTube, Movie 2K, Free TV Video Online, and Solar Movie.
One place for one to watch kids sports for free online would be at a website named Top Documentary films. One must choose the sports category once on the website.
NASA videos can be watched online at their official website. It is absolutely free. Alternatively, it can be watched at Top Documentary Films or Live Tv Cafe.
The popularity of a website is difficult to gauge, and typically would be a subjective measure. Some good documentary websites include Top Documentary Films, and Documentary Heaven. One can also view some of the PBS documentaries on the PBS Video site.
Some good free documentaries online include "The Story of Stuff," "Drone On" and "Gearing Up." You can learn more about these documentaries and watch them online at the Top Documentary Films website.
It's available on YouTube
Watch the documentary "Forks over Knives".