Miss Steenkamp, a model and law graduate, was locked inside a toilet when Pistorius fired four shots from a handgun through the bathroom door.
During his trial, the athlete maintained that he thought he was shooting an intruder and denied being jealous or aggressive during his relationship with the 29-year-old.
He was found guilty of murder and is serving a six-year sentence.
The model wearing the offensive outfit appears on the Amazon site equipped with sunglasses and a toy gun.
Alongside the online giant, a joke shop in Bedfordshire, is also selling the outfit. The Jokers' Masquerade shop in Luton gives a warning to anyone buying the costume, with their website stating: "If you are easily offended our Bad Taste Costumes may not be for you.
"In a world where political correctness has gone mad, we decided to be a little rebellious by compiling this collection of wonderfully inappropriate costumes specifically for those who like to push the boundaries."
Earlier this week an Anne Frank Halloween costume was removed from several online sites after Holocaust survivors complained it was in poor taste.
Originally sold on the website Halloweencostumes.com, the insensitive kids' costume is made up of a green beret, a blue coat and a brown satchel with a name tag on it like the kind evacuee children wore during World War II, reports The Sun.
The same costume is available to buy in Australia, where it is retailing for A$28.95 on the website Smiffys Australia Costumes.
It is described as a "World War II Evacuee Girl Costume."
