S rank items are only available in hard mode, past a certain point in the map. Personally, I found the stage to grind was Mishima Industries Biotech Research Station Ruins. If you progress through the level and beat the standard enemies, they have good drop rates. You'll know you've obtained a rank S item on hard when you see one of the treasures that looks white and different from your standard chest.
If you aren't aware how to unlock the stage, simply play Seahorse Grand Hotel and when you are given the choice to go left or right, simply go right. You'll know you've done this right when you face a kangaroo on your way to beating Christie.