Base game you can get most bdsm gear from it being put on you or taking it off someone else during *sex. Also randomly when you win a fight.
You can buy hypnovisors from Alex after progressing his quest line a bit.
You can buy only these two bdsm gear from the announcer. Zip tie cuffs and zip tie ankle cuffs.
*You can only recover restraints from others during sex.
You can use debug to give yourself restraints they will be put in your inventory.
You can activate debug by going to the main menu options menu and activating it. Then you can use it in game.