For free idol movies, you can visit YouTube, GyaO!, Nico Nico Douga, and all other sites as you wish.
If you want to purchase variety of Japanese Gravure-Idol movies with high quality by downloading, there is practically one choice.
In this area, DMM.com keeps a monopoly.
Free membership is essential to login.
The site is also suitable for downloading Japanese films including adults films, Japanese animes, and fanbooks.
Check the “Gravure Idol” page yourself. Unfortunately, unless you are a member, you cannot watch trailers.
The company announces that movies can be downloaded only inside Japan. It is very unfortunate.