You can buy them from iTunes or you can use a program to rip them from DVD to then upload to your phone.How do you download movies into iPhone?
I choose space over quality. I compress the movies down to 300-400 mb maximum and they are still watchable. Right now I have 4 feature length movies and four seasons of Aqua Teen Hunger Force and I have another 4 gigs of music.It depends on quality. Ive seen 300mb to 1gb a movie. 200 is watchable. It all depends on your preference.