Where exactly did the S&M look come from? By which I mean, tight black leather outfits, “gimp” masks with zippers, spiked collars, and all the other accessories that characterize modern S&M gear? Was there a specific time when this stuff came to be, or has it just evolved over the years? (If the latter, then what exactly did it evolve from?)
See older thread
What is the origin/history of submission/domination fetish wear?
The Marquis de Sade may have written extensively about sadism but I don’t think anything of the modern day S&M look is referenced in his books.
I blame the Victorians for it. De Sade has tons of nastiness but doesn’t read at all like any porn we’re familiar with. Only a specialist would willingly read it. Late Victorian (and Edwardian) porn not only could be reprinted today with no problem but often is. Victorian porn is full of leather and corsets. There’s also a whole subgenre of “sissfication” in which boys are made up to look like girls and be dominated.
Leather clothing also become associated with the outdoors in the U.S. and thereby picked up a macho aura. Bikers turned to leather early, probably after vets brought back leather jackets from WWI.
The Gay Leather Fetish History site has lots of pictures starting from the 1940s. I think they’re technically safe - no nudity, no overt sexual activity - but they’re not innocent either.
Female accessories probably came from Paris or Vienna, where prostitutes and entertainers dressed in costumes of corsets and high boots. Pictures of 1920’s designer fetish boots are similar to today’s.
Bette Page’s fetish photos from the 1950s are close to modern, although the bondage hoods and spiked collars for women would come later.
Much of the evolution probably came because of better manufacturing techniques that could create soft pliable leathers that could better be molded into face wear.
So no one source, no one time, no one country. Fetishists are extremely cosmopolitan. They get together in outsider groups all over the world and share ideas. Over time these has gotten more extreme and more commercialized. There are genres and subgenres in fetish today too. Don’t be caught dead wearing the wrong spikes after Labor Day. 
I thought of Weimar Germany too late. Post WWI Berlin was the epitome of decadence in Europe.
A bit of Googling brought up this contemporary drawing, [noparse]http://www.drsusanblock.com/editorial/weimarloveclub/WeimarBig/KamBond.jpg[/noparse]. It has the long leather slit dress, the high leather fetish boots, a whip, a muzzle, and a well-marked backside.
![http://www.drsusanblock.com/editorial/weimarloveclub/WeimarBig/KamBond.jpg[/noparse]](http://www.drsusanblock.com/editorial/weimarloveclub/WeimarBig/KamBond.jpg%5B/noparse%5D){kind=link}
The image comes from Dr. Susie Block’s Weimar Love Club, a definitely NSFW site
[noparse]Weimar Sex with Dr. Susan Block
I think you’d also have to add some degree of influence from Tom Of Finland who drew lots gay leather related stuff in the 40’s - 60’s that’s practically archetypal of modern SM fetish gear.
Tom of Finland’s work is pictured on The Gay Leather Fetish History site I cited.
As an aside, supposedly Heavy Metal’s affinity for the stuff was started (or at least was made popular) in the early 80s by Rob Halford, lead singer of Judas Priest who, it turns out, was gay.