Comcast plans to start selling movies for download and streaming through the cable operator’s set-top boxes and its Xfinity TV website, according to people with knowledge of the plan. The effort, which would offer a new path for Hollywood studios to generate revenue from films after they leave theaters, could start by the end of the year, the sources said. The initial offering will include a range of titles from several Hollywood studios that include new releases, older movies and some TV shows, one of the sources said. A Comcast spokeswoman declined to comment. Now, Comcast and other cable operators offer on-demand movies for free viewing or rental to watch during a specific time period. Movie studio executives are looking for ways to boost digital sales of movies as customers move away from purchases of traditional DVDs, cutting a lucrative source of revenue.