Obligatory shower scenes, sadistic wardens, lesbianism, catfights, breasts, low budget... hmmm, sounds like "Women in Prison" movies to me! The genre, sometimes disguised as pseudo-feminism, verges on sexploitation and can often be found with an "adult content" sticker on the box. They also vary from really bad to deliciously bad, but are almost always good for two hours of unadulterated adult entertainment.
Switchblade Sisters (1975)--Not so much "women in prison" as a girl gang flick (there is a jail with a butch lesbian warden scene), this movie will nevertheless keep your rapt and guffawing attention. Knives, chains, guns, roller skates, rape, treachery, power, leather, lust, high school, and boy gangs, Switchblade Sisters has it all.
Ilsa, The Wicked Warden (1978)--Ilsa, played by Dyanne Thorne, is the sadistic warden of a South American prison masquerading as a women's clinic "for sexually deviant women--nymphomaniacs, lesbians, and prostitutes." Great campy dialogue, good make-up and some truly horrific torture scenes.
Caged Heat (1974)--Jonathan Demme's directorial debut, though not particularly original, is still entertaining as hell. Good camera work lends to a passable plot that involves a psycho-sadistic prison doctor giving the hot! hot! hot! inmates shock treatment (and a little bit more) until they are walking zombies.
Girls in Prison (1994)--Just because the movie takes itself too seriously doesn't mean you have to. Starring Anne Heche and Ione Skye, this is your classic "girl framed for murder, girl goes to prison, girl finds lesbian friends who help her, girl released from prison, everyone is happy, even lesbian friends who remain in prison" type plot. Don't watch the last five minutes, though; it ruins the entire movie.