I suspect the studio supplies the clothes actors wear- they have large wardrobe departments for that sort of thing.
But movies are easily manipulated- in "Road House" there was a nice scene of Patrick Swayze getting out of bed nude and slipping into a pair of jeans freeball. When I use to see this movie you could see his whole backside. Now all you see is his back from the waist up. In fact, you don't even know that he was nude or put the jeans on freeball. Movies are edited this way all the time.